topdown/tokens: Fix verifying with multi-key JWKS

We were previously ignoring any keys beyond the first index in a set
of keys contained in a JWKS provided to the JWT builtins. We now will
attempt to use any of the keys found.

In addition we no longer raise an error if the key type doesn't match
the header. We will return the correct result as if the verification
failed (because it did..)

Fixes: #1901
Signed-off-by: Patrick East <east.patrick@gmail.com>