THRIFT-4838: Add unix socket support for Swift

[kinoroy]

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• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

What

Adds unix domain sockets as a low level transport option for the Swift library

Why

To improve Swift language support in Thrift.

This change will, for example, enable the use of the popular OSQuery library natively in Swift (currently only supports Obj-C++)

How

Extend the existing TSocketServer with a new initializer, taking a path input to a Unix domain socket path, and connecting to it. A new UnixSocket wrapper class was created for wrapping the different socket implementations in Linux and macOS.

Compatibility

All changes are additive or equivalent; there are no breaking changes.

Testing

Enables domain socket tests in the cross-test suite for Swift.

Some cross-tests are failing due to other languages not implementing the UUID type or their implementation doesn't match the spec.
After rebasing on master it seems the cross-tests also seem to not be running right now in CI due to a failure to build the go library