App access JWT improvements

[r0mant]

Couple improvements/fixes for app access' JWT integration. Specifically:

• Include iat (IssuedAt) claim which wasn't present before. Some apps that support JWT auth require it.
• Add ability to inject JWT in any header via {{internal.jwt}} template variable. With this change, you can for example have the following app configuration to pass JWT in Authorization header:

apps:
- name: "example"
  uri: http://localhost:9200
  rewrite:
    headers:
    - "Authorization: Bearer {{internal.jwt}}"

These changes are required to support JWT authentication in more applications. For example, with these changes I was able to successfully configure JWT authentication/authorization with ElasticSearch:

https://www.elastic.co/guide/en/elasticsearch/reference/current/jwt-realm.html

[zmb3]

bot.

[github-actions]

@r0mant See the table below for backport results.

Branch	Result
branch/v9	Create PR