Get-Only Access of Source Properties from Connection Pools

[calebkleveter]

This PR adds a .sourceInfo property to the EventLoopConnectionPool and EventLoopGroupConnectionPool types. The .sourceInfo property is of type SourceInformation<Source>.

The SourceInformation type is dynamicMemberAccessible, where the subscript takes in a KeyPath<Source, Property>. This allows get-only access to properties on the private .source property in the connection pool types that you can access in your current scope (so public or internal depending on whether you are in the module that defines the Source type).

Checklist

• [x] Circle CI is passing (code compiles and passes tests).
• [x] There are no breaking changes to public API.
• [x] New test cases have been added where appropriate.
• [x] All new code has been commented with doc blocks ///.
• [x] The linuxMain.swift is regenerated using swift test --generate-linuxmain

[calebkleveter]

I just fixed that issue, so the CI should actually run now. 😄