platform.md: Define RemoteApisSocketPath for nested remote execution

[juergbi]

This defines RemoteApisSocketPath as a new platform property to optionally provide remote commands access to the Remote APIs via a UNIX socket at a specified path.

This is intended to e.g. support running Bazel as a remote command while allowing the remote Bazel to then use nested remote execution or remote caching for individual actions. Or to use recc to speed up compilations within a package build that is also using remote execution (e.g. recc in BuildStream).