chore: add gemini cli extension configuration (#1727)

## Description

Create a Gemini CLI extension for the repo to recommend other
extensions.

Notes
* Since the repo has releases. The new release will trigger an extension
update rather than a version update in the JSON.
* MCP-TOOLBOX-EXTENSION.md is used in order to keep GEMINI.md free for
the project context.

## PR Checklist

> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [ ] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [ ] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

---------

Co-authored-by: Kurtis Van Gent <31518063+kurtisvg@users.noreply.github.com>

.github/release-please.yml

@@ -37,4 +37,5 @@ extraFiles: [
     "docs/en/how-to/connect-ide/postgres_mcp.md",
     "docs/en/how-to/connect-ide/neo4j_mcp.md",
     "docs/en/how-to/connect-ide/sqlite_mcp.md",
+    "gemini-extension.json",
 ]