test(handlers): test combinations of payload and Content-Type media types during a manifest push

This MR adds test coverage for all possible combinations of payload and Content-Type media types during a manifest push. This includes tests for Docker, OCI, and unknown media types.

Please note that some of the test results are not what we would expect. For example, we can see that currently, we're fallbacking to the Docker schema 1 parser when no media type is provided, which results in a weird error message of missing signature key. This is a problem. However, the intent of this MR is not to fix that but rather to create a representation (with tests) of the current behavior. We can then iterate on this and analyze case by case, defining the best way to move forward (i.e. what should be the response for each one of these scenarios/combinations).

Tests will fail until !711 (merged) and !710 (merged) are merged and this rebased.