Add RequestIdentityMode to PDP requests

[codingllama]

Add RequestIdentityMode to PDP requests, allowing callers to more easily query about 3rd parties or themselves.

Without this feature callers would have to always form a complete request identity, which is
(error-prone) busywork when querying about self and needless complexity when querying about 3rd parties (whose identities those tools shouldn't need to or even be able to form).

Querying about 3rd parties is subject to authz controls, naturally.

#49837

[codingllama]

Thanks for the reviews. @fspmarshall has an alternative design in mind, so I'll hold this until his has a chance to be reviewed.

[codingllama]

Thanks for the reviews. @fspmarshall has an alternative design in mind, so I'll hold this until his has a chance to be reviewed.

Friendly ping @fspmarshall.

[codingllama]

Closing as it's likely we'll either go with #50559 or some other in-between approach we talked about out-of-band.