Add option on remote caches to be read-only?

[bartdesmet]

It'd be useful to have a "read-only" flag for remote caches, similar to ccache's support for a read-only=true flag that can be specified on a per-remote basis. See https://ccache.dev/manual/4.9.1.html#_attributes_for_all_backends.

Our scenario is a bunch of CI machines that run builds, including clang-tidy checks. These all use a common cache using clang-tidy-cache-server, which greatly speeds up the CI pipelines (over 98% gains). Developers would also like to benefit from this cache for local builds, but we wouldn't want to pollute the cache as they're making local iterations to their code, often in a tight development loop with repeated builds using clang-tidy.

If there's no objection, I'm happy to prepare a change, potentially with CTCACHE_[S3|GCS|HOST]_READ_ONLY flags that default to 0 but can be set to 1. The effect would be for store_in_cache functions to simply return.

[matus-chochlik]

I don't see why not off the top of my head, I'd be happy to merge a PR with the described changes.