Add `pants.remote-cache.toml` (#11394)
Add `pants.remote-cache.toml` (#11394)

Toolchain is making improvements to remote caching, and this change facilitates setting that up to be used.

There are two contexts where we often use remote caching:

1) Locally, typically by spinning up a local server.
2) In CI

These have different settings, such as which server to point to. Instead of having two separate config files, we leave out the context-specific settings. Developers can use `.envrc` to automatically set them, and in CI, we can set `.travis.yml` appropriately.

[ci skip-build-wheels]
[ci skip-rust]
benjyw/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,jsirois/pants,jsirois/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,jsirois/pants