Add single-get multi IDP support for passive mode

[npm1]

Adds support for multiple IDPs on passive mode by:

• Not rejecting if the list passed is not of size 1.
• Still rejecting if the list passed is not of size 1 and mode is active (it is possible this PR is later updated to remove this restriction)
• Performing some per-IDP checks first, and computing a map based on the results
• Showing UI for the relevant IDPs based on the results from the computations: optional UIs for logged out IDPs, mandatory UI for mismatch IDPs, and all accounts for IDPs that returned some accounts.
• Add a configURL to IdentityCredential so the RP knows which IDP the credential corresponds to.

Also some unrelated errors noticed while drafting this PR are fixed.

Relevant issue: w3c-fedid/multi-idp#2

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