finish PR #100: update "request a credential" algorithm

[equalsJeffH]

finish PR #100: update "request a credential" algorithm to accept [[DiscoverFromExternalSource]] returning an algorithm, which "request a credential" then runs in a task. We need to do this since [[DiscoverFromExternalSource]] in webauthn returns such an alg. This will match how credman's "create a credential" presently works.

See also:

• accessing settings object from in-parallel steps? #92 (overall issue)

Plus these issues (which are more-or-less related):

• Make [[Create]] consistently create its Credentials from a task #192
• Do not reference EventLoop objects from "In parallel" sections #177
• NULL or DOMException #148

(there may also be other related issues, if so please add them here)