Add strict/coercion mode

[ixunio]

class MyJWTClaims(JWTClaims):
    # redefine 'sub' as required integer
    sub: int = Field(default=...)

MyJWTClaims(**{"sub"="123"})  # not failing

Default behavior should reject coercion for standard claims (or all of them?)

Also for the json.dumps() with non str keys

Related: jpadilla/pyjwt#1082

EDIT: not needed, standard claims are str safe for the timestamps which has already an issue opened re the int/float serialization and validation