Provide SwiftFormat toolchain

@cgrindel and I are discussing the best way to provide formatting capabilities for Bazel swift users.

The first facility required is a Bazel repository rule that fetches a prebuilt formatter binary and exposes it as a toolchain constrained on the execution platform. It seems like rules_swift could be a good place to host that code, as it already exposes other swift tools. Of course this new toolchain would be orthogonal to the swift compiler, and only fetched if the user invokes actions which require it.

I'm not sure which formatter(s) should be included. These two seem like the obvious candidates from a quick Googling:
https://github.com/nicklockwood/SwiftFormat
https://github.com/apple/swift-format

@allevato @keith do you have any strong opinions on whether such a toolchain could be contributed here, or if you'd prefer it live in some other repo?

Start by reading rules_swift's existing Swift tool integrations and Bazel repository-rule and toolchain documentation; the issue names no files or tests. First resolve whether SwiftFormat or swift-format belongs here, then define a repository rule that fetches the selected prebuilt formatter and exposes a platform-constrained toolchain fetched only when needed.

Written by the indexing model from the issue text.