Add context.Context support to API methods

[optistar]

Adds context.Context to HTTP requests for cancellation/timeouts, as requested in #108.

• Introduces ...Context variants for each API method, taking ctx as first arg.
• Original methods preserved for compatibility, delegating to ...Context with context.Background().
• If ...WithContext is preferred, adjust name_suffix="Context" and 'Context(' in the generator.
• Avoids breaking changes (e.g., no ctx in arg structs).
• The signature of dropbox.context.Execute is changed to take ctx, based on the assumption that this method is internal.
• In Execute, the call to http.NewRequest is replaced with http.NewRequestWithContext.
• No doc comments is set on ...Context methods; something like // UploadContext is the context-aware version of Upload. could be added.

Generated code is not updated by this commit. Example of impact:

Before:

func (dbx *apiImpl) Upload(arg *UploadArg, content io.Reader) (res *FileMetadata, err error) {
  // implementation
}

After:

func (dbx *apiImpl) Upload(arg *UploadArg, content io.Reader) (res *FileMetadata, err error) {
    return dbx.UploadContext(context.Background(), arg, content)
}

func (dbx *apiImpl) UploadContext(ctx context.Context, arg *UploadArg, content io.Reader) (res *FileMetadata, err error) {
    // implementation with ctx in Execute
}

Closes #108.

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