Generic `--host_opt` flag

One thing we see come up a lot is needing to pass options that we pass to our build differently for the host configuration. In some cases bazel has a flag for this like `--host_swiftcopt` / `--swiftcopt` etc. In other cases, for whatever reason, bazel does not yet have a flag for the use case, such as https://github.com/bazelbuild/bazel/pull/13001 or `--host_features` and how `--features` applies everywhere.

I think it would be useful for there to be a generic `--host_opt` flag with syntax such as `--host_opt=--normal_opt=value` that allowed you to pass through arbitrary bazel options to the host configuration.

This opens the door for much more API usage for the host configuration, but I think that could be preferred to the trade off of trying to add a host flag for ever applicable option.

Review the existing --host_swiftcopt/--swiftcopt and --host_features/--features behavior, along with Bazel's option parsing and host-configuration handling. Done means the proposed --host_opt=--normal_opt=value syntax is defined, arbitrary supported options can be passed to the host configuration, and behavior is covered by appropriate tests.

Written by the indexing model from the issue text.