Added P-256 to decentralized-identity/did-jwt

Hello all,

I have been doing some work modifying the did-jwt library so that it adds a
P-256 support with an ES256 signer. I did quite a bit of refactoring of the
library, although since all the tests pass I believe it should function as
before for previous uses.

https://github.com/decentralized-identity/did-jwt/pull/212

Does this code seem reasonable? Is there an appropriate way/means to garner
a review? It seems to be included as a dependency in several other
repositories.

fwiw, I imagine that ES384 and ES521 and RSA and .... could more easily be
added given the refactoring. Maybe these signers will be more useful to
others. In the future, I may consider adding them since I saw some talk
about supporting legacy HSM,
https://www.iana.org/assignments/jose/jose.xhtml

TODO: It might need a package structure more akin to:
https://github.com/StableLib/stablelib/tree/master/packages if the
repository gets too big?
See: https://github.com/decentralized-identity/did-jwt/issues/170

-Brent Shambaugh

I think I left some contact information below, and I recall signing some
form for the DIF awhile back.