feat(internal/librarian/golang): support configurable tool dependencies in librarian.yaml

[sofisl]

The current install implementation for golang has a hard-coded list of modules and versions to install. This makes it difficult to update the versions of Go tools used for GAPIC generation per repository. This change adds support for defining Go tool dependencies in librarian.yaml under the tools property, similar to other languages. It introduces the GoTool struct in config, updates the installer to fetch dependencies from configuration (falling back to defaults if not defined), and regenerates the configuration schema documentation.

Fixes #5377

[gemini-code-assist]

Code Review

This pull request introduces support for Go tool installation via the librarian.yaml configuration, including schema updates, configuration struct additions, and implementation of the installation logic with a fallback mechanism. Feedback focuses on handling cases where a tool version is not specified to avoid invalid go install commands, and removing the unused Exec field from both the configuration and documentation to adhere to the YAGNI principle.

[sofisl]

@noahdietz, here are a few decisions/assumptions I made in this PR (feel free to correct me!):

• Left the hardocded tools as a fallback
• Included the Exec property as an explicit binary name, that should override the goimports binary.

some more questions about exec - do you want it to be derived if not specified? (according to #5201)?

[JoeWang1127]

Left the hardocded tools as a fallback

Should we file an issue as a follow up to remove the hardcode tools once the tool section is introduced to librarian.yaml in google-cloud-go?

[sofisl]

Thanks for all the indepth reviews!