Allow hs2019 algorithm in signing and verifying (rebased, improved, tests, squashed)

[ashpieboop]

Resolves #106

From #105:

This allows the recommended use of "hs2019" as algorithm, that effectively hides the used algorithm from the signature to avoid attacks, see Appendix E.2 in https://tools.ietf.org/html/draft-cavage-http-signatures-12.

For signing there is an option hideAlgorithm that defaults to false and if true writes "hs2019" into the authentication header.

For verifying one has to pass an overriddenAlgorithm to verify that basically overrides the algorithm "hs2019" for verification. The overriddenAlgorithm has to be determined by metadata of the clientId (after parsing the signature the clientId is known).

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I removed the manual algorithm override from original PR as it's supposed to be determined by the key metadata (aka public key type) for hs2019.

Also, hash algo is supposed to be sha512 so it is forced in validateAlgorithm().