[Webhook Connector] Handle missing ALG key in JWT Auth

[johnBgood]

Support issue
https://jira.camunda.com/browse/SUPPORT-24829
https://jira.camunda.com/browse/SUPPORT-25323

Describe the Bug

The alg key is optional in the JWK Specification. Currently, we expect this key to be there or it throws an NPE.

This works with Keycloak as the alg key is present, but with Entra for instance it might not work.

The fix should be backported to 8.6.

Steps to Reproduce

1. Use EntraID
2. Use a Webhook StartEvent with JWT Auth
3. Send a request
4. A 500 error is returned

Expected Behavior

A 200 response should be returned as usual.

[sbuettner]

More information was requested in the incident slack channel: https://camunda.slack.com/archives/C083JMZGUS2/p1734344977153109

[sbuettner]

The Connector should use the alg value which is part of the JWT headers as a fallback:

{
  "typ": "JWT",
  "alg": "RS256",
  "x5t": "...",
  "kid": "..."
}