Add support for `--repo_contents_cache`

[brentleyjones]

🚀 feature request

Description

Repository rules should use the new ctx.repo_metadata function if available (8.3.0+, checkable by hasattr). This can speed up cold startups, like ephemeral CI with cache volumes.

References:

• Add support for --repo_contents_cache bazel-gazelle#2119
• feat: add support for --repo_contents_cache cgrindel/rules_swift_package_manager#1705
• https://github.com/bazelbuild/bazel/blob/7827acc1287b1861c89d129658efd81b7209fffa/tools/build_defs/repo/git.bzl#L206

[aignas]

If anybody is eager to get a start on this, feel free to do this. I think ideally this can be approached separately for the python toolchains and the PyPI packages. Right now I am busy stabilizing experimental-index-url but I am happy to review incoming PRs on this.