Improve CI pipeline with automated builds and tests for the primary branches (v0.34.x, v0.37.x and main)

[p-offtermatt]

Currently, the automated build and test infrastructure for CometMock is very limited.
This allowed, for example, a commit that caused a build error to sneak by (see #14). In the interest of avoiding such errors, a few things should be done:

• On each commit, build CometMock on the primary branches #17
• On merged PRs with the label 'backport-v0.34.x' and 'backport-v0.37.x', PRs are backported to the respective branch automatically, i.e. a PR is opened automatically. #22
• On each commit, run a test suite against a suitable version of the Cosmos SDK simapp #65
• On each commit, run a test suite against a suitable version of Interchain Security #63
• On each commit, run a test suite against a suitable version of Penumbra #64