Make frame type a typed constant

[jhendrixMSFT]

Added fmt.Stringer support. Removed duplicate definition from mocks package.

• [ ] The purpose of this PR is explained in this or a referenced issue.
• [ ] Tests are included and/or updated for code changes.
• [ ] Updates to CHANGELOG.md are included.
• [ ] MIT license headers are included in each file.

[richardpark-msft]

Ooooh, this is an interesting one! Reviewing...

[jhendrixMSFT]

Just some pretty-printing that the logging clean-up will take advantage of.