RFE: expose delegated metadata to client application

EDIT: The overall issue is described in detail in https://docs.google.com/document/d/1rWHAM2qCUtnjWD4lOrGWE2EIDLoA7eSy4-jB66Wgh0o . The suggestion here is roughly the Metadata role (file) as search index solution in the document.

Assume a setup like this (this is what we expect a community artifact repository like PyPI to look like if it uses developer signatures with TUF):

• a specific project/product team controls a delegated metadata
• TUF clients want to know details of all of the artifacts in this metadata (to e.g. figure out which versions of an artifact are available)

Currently there is no way for the client application to get the whole metadata content from ngclient. We could provide a call much like get_targetinfo() that instead of the TargetFile would return the Targets object where the target search ended:

def get_targets_metadata(target_path: str) -> Targets
    """returns a Targets object of the metadata where the search for target_path terminated"""

This is not applicable to every TUF repo:

• it requires a "contract" between repository and client: client has to know of a target_path that is delegated to the correct metadata -- in the pypi example it could be e.g. the PyPI project name
• this is only useful if all "related" target files are listed in the same metadata

But with those assumptions the client can now easily get not just the list of target files it's interested in but also any custom metadata embedded in the targets metadata.

I've not thought through all the cases (what happens if there is no targetpath match? what if there is no terminating delegation?) but I think this is something we could consider implementing

Start by reading ngclient's existing get_targetinfo() flow and the linked design document, especially the Metadata role as search index proposal. Trace how delegated metadata is searched and determine the expected behavior for unmatched target paths and non-terminating delegations. Done means the client can expose the relevant Targets object and has coverage for the agreed edge cases.