[Feature request] Add per_file_swiftcopt / host_ per_file_swiftcopt flags

Thes `per_file_copt`[1] and `host_per_file_copt`[2] flags are super useful for customizing C/C++ compiler flags based on source path. A common use case is to e.g. optimize external dependencies or suppress warnings for sources in `external/`. It would be useful to have an analogue for `swiftc` invocations provided by `rules_swift`.

[1]: https://bazel.build/reference/command-line-reference#flag--per_file_copt
[2]: https://bazel.build/reference/command-line-reference#flag--host_per_file_copt

Start by reading the linked Bazel per_file_copt and host_per_file_copt references, then trace how rules_swift provides swiftc invocations. Done means rules_swift exposes equivalent per-file and host per-file Swift compiler flag behavior for source paths, with coverage for the supported invocation paths.

Written by the indexing model from the issue text.