Add Github Actions to build & test

[zmichaels11]

Changes

• Runs GitHub Action to build & test 4x a day and on pull requests

Testing

• Tested on local fork: https://github.com/zmichaels11/rosbag2_bag_v2/pull/3 fork deleted post merge

[zmichaels11]

Its set to hourly to quickly pick up any breakage from upstream dependencies and also to track flakiness.

Anyway, I might make it build & test less frequently if build&test begins to overlap. Its currently taking close to 50min.

Edit: I'll probably bump it up to 6hrs after this goes green so we're not spamming the CI. Its set to run 4x a day.