Support `GLOBAL` scope for the `tidb_low_resolution_tso` variable

[cfzjywxk]

Feature Request

Is your feature request related to a problem? Please describe:

Support GLOBAL scope for the tidb_low_resolution_tso variable.

For scenarios where strong consistency read is not required, using stale read can significantly save resource overhead and achieve better performance.

When tidb_low_resolution_tso supports the global scope, enabling stale read for such scenarios becomes even simpler.

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: