Better integration test support for changing schemas

[mikewilli]

The integration tests pull in metric-hub's main branch, which can cause issues when changing schemas in a way that is not backwards compatible.

Ideas

• add a branch specific to integration tests that only gets pushed to manually when the PR owner knows they are making an incompatible change
  • slight tweak: we could push to this branch automatically on every PR

Context: #1415 and #1417 needed to be split into two PRs because the integration tests were failing when trying to change a value that gets pulled in and validated by ConfigCollection.

Decision at time of writing is to do nothing because:

• this issue very rarely comes up
• the ideas feel like a bit of a workaround and the effort to implement outweighs the benefit at the moment

Documenting for now, and if the issue recurs more frequently we can reconsider this decision.