tuf's Metadata API: Targets.get_delegated_role() is missing input validation

Summary

The security of both a TUF client and repository implementations depend on the concept of trusted Metadata objects verifying the signatures over other Metadata that it delegates to. This verification process uses `Targets.get_delegated_role(delegated_role: str)` to find the delegation information.

`tuf.api.metadata.Targets.get_delegated_role()` should ensure that the given `delegated_rolename` is actually a name of a role that is delegated by that Targets, but in the case of "succinct delegation" this does not happen.

`tuf.ngclient` users are **not** impacted but direct users of `tuf.api.metadata` could be impacted.

### Impact

If an attacker can make a Metadata API user run `Targets.get_delegated_role()` so that 1. the `Targets` uses succinct delegation 1. the `delegated_role` argument is not actually delegated by the `Targets`

the result will be incorrect.

This also means that if an attacker can make a Metadata API user run `Targets.verify_delegate()` or `Targets.get_verification_result()` so that 1. the delegating `Targets` uses a succinct delegation 1. the `delegated_role` argument is the name of some unrelated Metadata 1. that other Metadata is correctly signed by the keys defined in the succinct delegation

the result would be a successful verification even though the `Targets` in question does not actually delegate to `delegated_role`.

The impact is estimated to be low for following reasons:

* This cannot impact a TUF client that implements the client workflow as specified since the delegated role name is not an input but is collected from the (trusted) delegating Targets itself * Actual signature verification is not bypassed: The verified metadata _must_ still be correctly signed by the keys specified in the delegating role. * The described situations are somewhat hypothetical: there does not seem to be any reason for a python-tuf user (whether client or a repository) to use `tuf.api.metadata` in this way.

All users of `tuf.ngclient` are specifically **not** impacted. Users of `tuf.api.metadata` could be impacted if they use succinct delegations in a way described above.

### Patches

A fix is available in python-tuf 3.1.1 as commit 77cb66bc and in later releases as commit eb4834d9._

### Workarounds

`tuf.api.metadata` users should only call `Targets.get_delegated_role()`, `Targets.verify_delegate()` or `Targets.get_verification_result()` with `delegated_role` argument that is known to be delegated by the `Targets` in question.