Warmstart automation

[le1nux]

What does this PR do?

This PR simplifies warmstarts from a checkpoint by adding a warmstart entrypoint.
We pass the warmstart config and the checkpoint info file to the endpoint. The checkpoint info file contains the path information for the model and optimizer checkpoint. The checkpoint path is injected into the config via a pydantiv env resolver.

Checklist before submitting final PR

• My PR is minimal and addresses one issue in isolation
• I have merged the latest version of the target branch into this feature branch
• I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• I have run a sample config for model training
• I have checked that all tests run through (python tests/tests.py)
• I have updated the internal changelog (CHANGELOG_DEV.md)

[fromm-m]

great work!

[flxst]

Looks good, but I think the end2end test might not be sufficient, see my comment.

[flxst]

LGTM! Added a few comments that should perhaps be addressed, nothing serious though.

[flxst]

I wonder if we should move this line to the end of the shell script, right above the final "Finished warmstart example" message. This would clean up the temporary data right after the test, instead of cleaning up the data of a previous test at the beginning. (This is what is done in the case of the getting started example test as well).