Support `ImpressionSourceType`

[yunapotamus]

ImpressionSourceType changes

• Add support for ImpressionSourceType enum in impression logging classes.
• Regenerate Swift protos with latest schema that supports this enum.
• Plumb through support for this enum in MetricsLogger.
• Add ImpressionConfig protocol to provide configuration interface to ImpressionTracker and ScrollTracker.
• Add test that this value gets logged in protos.

Rename ImpressionLogger --> ImpressionTracker

• Rename the class and related methods/variables to better conform to ViewTracker and ScrollTracker naming.
• Move impression tracking classes into their own directory.

Cleanup

• Change a few code blocks in comments to better work with Xcode 13 beta.
• Divide ImpressionTracker and MetricsLogger+Helpers into more extensions to group class functionality.