Prepare remote::remote_cache::CommandRunner for other providers

[huonw]

This does preparatory refactoring towards #11149, by adjusting remote::remote_cache::CommandRunner in a few ways to make it easier to plop in new 'providers':

• package the various options for creating a provider/command runner into structs, and a bunch of mechanical refactoring to use those structs
• explicit tests for the REAPI provider

This continues #19424, but, unlike that one, doesn't refactor remote_cache_tests.rs to (mostly) use in-memory providers, as those tests are significantly more complicated, with many more services than just the get/update caching provider and I don't think it strictly blocks #11149.

After this, the next steps towards #11149 will be:

1. implement new providers, with some sort of factory function for constructing the appropriate provider
2. expose settings in pants.toml to select and configure those new providers