Add CredentialsManagement public key options and other missing bits

[autonome]

Date- and spec-wise this seems to all jive w/ the original feature, no baseline changes.

The only weird bit is that Caniuse seems incorrect here, unless they're only counting "support" as also supporting some of the optional parameters (unlikely, and would be strange): https://caniuse.com/credential-management. But that doesn't change this PR, just a side note.

[autonome]

In the spirit of "small patches for higher velocity", not going to blow this up into larger Credentials patch. Instead will do it in series of smaller chunks+refactors.

[ddbeck]

Hmm, I find this to be a hard area to understand (we had to call in help from a Chrome engineer on #868). Do the extensions to create and get not belong with the webauthn feature? That's where most of the PublicKeyCredential stuff lives already. If not, why not?

[autonome]

Hrm, I did not search BCD keyspace first... yeah this set of features is sprinkled all over. Moved these to the right home at first glance, but need to look more, flipping back to draft.

[autonome]

Ok, checked spec, and reviewed the various versions these additions shipped in - these do not justify a separate feature, and don't change baseline status of either webauthn or fedcm. Requesting review from @nsatragno as well, per the original feature addition.

Update: Looks like I don't have permissions to request review from individuals 🫤

[ddbeck]

I have one minor suggestion. It'd be nice to get Nina's subject-matter expert review on it, but I have pretty high confidence in this one if she doesn't get to it by the end of the week.