.Net: Expose GeminiKernelFunctionMetadataExtensions (#12515)

### Motivation and Context
`GeminiToolCallBehavior` exposes the method `EnableFunctions` to enable
filtering which Kernel Functions are available for auto invoke. However,
there's no way to construct a `GeminiFunction` instance since the
constructor is internal.

I saw there's an extension to build a `GeminiFunction` from a
`KernelFunctionMetadata`, but it's under the test project. It seems that
it was supposed to be exposed from the library, since there's also a
test file for `KernelFunctionMetadata`.

This way, it will be possible to use
`GeminiToolCallBehavior.EnableFunctions` to explicitly specify which
tools can be auto-called by the Google connector.

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### Description

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- Move GeminiKernelFunctionMetadataExtensions from
`SemanticKernel.Connectors.Google.UnitTests` to
`Microsoft.SemanticKernel` namespace.
- Remove `FunctionMetadataAsGeminiFunction` and start using
`GeminiKernelFunctionMetadataExtensions` instead.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Author: JanainaLudwig

dotnet/src/Connectors/Connectors.Google.UnitTests/Utils/GeminiKernelFunctionMetadataExtensions.cs renamed to dotnet/src/Connectors/Connectors.Google/Extensions/GeminiKernelFunctionMetadataExtensions.cs

@@ -1,10 +1,9 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System.Collections.Generic;
-using Microsoft.SemanticKernel;
 using Microsoft.SemanticKernel.Connectors.Google;
 
-namespace SemanticKernel.Connectors.Google.UnitTests;
+namespace Microsoft.SemanticKernel;
 
 /// <summary>
 /// Extensions for <see cref="KernelFunctionMetadata"/> specific to the Gemini connector.

dotnet/src/Connectors/Connectors.Google/GeminiToolCallBehavior.cs

@@ -127,50 +127,11 @@ internal override void ConfigureGeminiRequest(Kernel? kernel, GeminiRequest requ
             // Provide all functions from the kernel.
             foreach (var functionMetadata in kernel.Plugins.GetFunctionsMetadata())
             {
-                request.AddFunction(FunctionMetadataAsGeminiFunction(functionMetadata));
+                request.AddFunction(functionMetadata.ToGeminiFunction());
             }
         }
 
         internal override bool AllowAnyRequestedKernelFunction => true;
-
-        /// <summary>
-        /// Convert a <see cref="KernelFunctionMetadata"/> to an <see cref="GeminiFunction"/>.
-        /// </summary>
-        /// <param name="metadata">The <see cref="KernelFunctionMetadata"/> object to convert.</param>
-        /// <returns>An <see cref="GeminiFunction"/> object.</returns>
-        private static GeminiFunction FunctionMetadataAsGeminiFunction(KernelFunctionMetadata metadata)
-        {
-            IReadOnlyList<KernelParameterMetadata> metadataParams = metadata.Parameters;
-
-            var openAIParams = new GeminiFunctionParameter[metadataParams.Count];
-            for (int i = 0; i < openAIParams.Length; i++)
-            {
-                var param = metadataParams[i];
-
-                openAIParams[i] = new GeminiFunctionParameter(
-                    param.Name,
-                    GetDescription(param),
-                    param.IsRequired,
-                    param.ParameterType,
-                    param.Schema);
-            }
-
-            return new GeminiFunction(
-                metadata.PluginName,
-                metadata.Name,
-                metadata.Description,
-                openAIParams,
-                new GeminiFunctionReturnParameter(
-                    metadata.ReturnParameter.Description,
-                    metadata.ReturnParameter.ParameterType,
-                    metadata.ReturnParameter.Schema));
-
-            static string GetDescription(KernelParameterMetadata param)
-            {
-                string? stringValue = InternalTypeConverter.ConvertToString(param.DefaultValue);
-                return !string.IsNullOrEmpty(stringValue) ? $"{param.Description} (default value: {stringValue})" : param.Description;
-            }
-        }
     }
 
     /// <summary>