Add HS384 and HS512 support

[ryanchapman]

Add support for HS384 and HS512

Example envoy config that uses the HS384 key secret (to anyone reading this, your key should be at least 48 bytes (384/8=48), 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\":\"HS384\"}]}"
        forward: true
        forward_payload_header: "plain-authorization"
    rules:
    - match:
        prefix: /protect
      requires:
        provider_name: sample_jwt_provider

Example envoy config that uses the HS512 key secret (to anyone reading this, your key should be at least 64 bytes (512/8=64), 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\":\"HS512\"}]}"
        forward: true
        forward_payload_header: "plain-authorization"
    rules:
    - match:
        prefix: /protect
      requires:
        provider_name: sample_jwt_provider