feat: add json schema generation for CasConfig via schemars

[Pegasust]

Description

Add JSON schema generation as a runnable binary target

cargo run --bin build-schema --features dev-schema --package nativelink-config

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update: I'm unsure if this is needed, can amend to this PR if needed.

How Has This Been Tested?

I made this change and just run cargo run --bin build-schema --features dev-schema --package nativelink-config, everything builds as expected and a //nativelink_config.schema.json is created.

I eyeballed the generated schema everything looks good enough for me, except for some cases like StoreRef or GrpcEndpoint would benefit from a soft validation regex, and/or some schema documentation bit that it has deserialize_with = "convert_string_with_shellexpand". Nevertheless, I shoulder upon existing docstrings for its detailed documentation and choose to not pursuit further modification to wordings I don't have deep knowledge upon.

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally: Probably not needed, was not in capacity to set this up locally. I gated my change under crate-local nativelink-config dev-schema to ensure this doesn't break existing workflow. This commit does add a little SBOM burden.
• PR is contained in a single commit, using git amend see some docs

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This change is 

[CLAassistant]

All committers have signed the CLA.

[aaronmondal]

Nice! Something similar has been in-flight for a while in #1477. That PR contains many changes which are slowly making it into main.

One standalone usecase I could think of regarding schema generation (other than the CRD generation from the above PR) is that it makes it straightforward to identify breaking changes. Could you create a bazel target (so that we can run something like bazel run nativelink-config//:generate-schema instead of the cargo invocation) and a diff_test to check whether the schema has changed? This is similar to what we do in the nativelink-proto directory to detect changes to generated gRPC sources.

Reviewable status: 0 of 2 LGTMs obtained, and 0 of 8 files reviewed