Add HS256 support

[ryanchapman]

Our company uses HS256 for JWT verification in a system, and we'd like to be able to use Envoy and Istio, so this PR adds support for that algorithm.

Example envoy config that uses the HS256 key secret (to anyone reading this, your key should be at least 32 bytes (256/8=32), not 6):

http_filters:
- name: envoy.filters.http.jwt_authn
  config:
    providers:
      sample_jwt_provider:
        issuer: sample.com
        local_jwks:
          inline_string: "{\"keys\": [{\"kty\":\"oct\",\"k\":\"c2VjcmV0\",\"alg\":\"HS256\"}]}"
        forward: true
        forward_payload_header: "plain-authorization"
    rules:
    - match:
        prefix: /protect
      requires:
        provider_name: sample_jwt_provider