Add functions spm unit and build tests

[jrcrespoh]

Tests ran successfully with watchos

pod lib lint fails on travis for watchos, but succeeds locally. Disabling test for now, pending GHA update to Xcode 12.5+

[ncooke3]

Nice! I think we should add watchos to the each matrix in functions.yml Here's one such matrix.

[google-oss-bot]

Coverage Report

Affected SDKs

No changes between base commit (c54fad2) and head commit (c7c29d7).

Test Logs

• Base (c54fad2f): https://github.com/firebase/firebase-ios-sdk/actions/runs/1101865930
• Head (c7c29d74): https://github.com/firebase/firebase-ios-sdk/actions/runs/1137185466

[paulb777]

Looks like pod lib lint does not work for watch OS. https://travis-ci.org/github/firebase/firebase-ios-sdk/jobs/774699071

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[ncooke3]

This LGTM 👍 . Added one comment about SwiftPM excludes as I havent seen that issue before

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[jrcrespoh]

What should be done about incremental code coverage reports? I imagine these should not surface on tests?

[paulb777]

@granluo is working on the incremental coverage reporting separately.

@jrcrespoh This PR should be good to resolve and merge here.