Merge "byte store" and "action cache" provider options

[huonw]

This replaces uses of RemoteCacheProviderOptions with RemoteStoreOptions (previously RemoteOptions): they're very similar, and only separate because they were previously in very different parts of the codebase, but this has changed with #19958.

This makes a few additional changes to make the merging smoother:

• removes the "capability cell" sharing optimisation, which enabled sharing of the GetCapabilitiesRequest gRPC request result between the CAS and the remote execution providers, if both were being used with the same address. I thought that optimising one-time (on start-up) requests from 2 to 1 wasn't worth the complexity of plumbing it through these more generic options, but can restore it if it is.
• renames various fields

This does meant that there's a few options that are seemingly irrelevant to the action cache providers, e.g. "batch API size limit" is meaningless. I think this is fine: I think it's fundamental to supporting multiple providers that there'll be a grab-bag of options, which some providers support and others do not.

The commits are individually reviewable.

(This is one point of #19902.)

[tgolsson]

Nice!

[stuhood]

Looks good: thanks!