OIDC: Implement PKCE extension

[romanstrobl]

PKCE will be implemented as an optional layer of security on top of OIDC activation.

Additional authorization code request parameters added by mobile application once PKCE is implemented:

• code_challenge=CODE_CHALLENGE
• code_challenge_method=S256

The server will need to obtain the code verifier value, so that the token can be received with PKCE protection.

The activation request will be updated to contain the codeVerifier.

  "identityAttributes": {
      "method": "oidc",
      "clientId": "1234567890abcdef",
      "code": "1234567890abcdef",
      "nonce": "qfT4K8p7R5G3XY7zcm3HAw==",
      "codeVerifier": "G3hsI1KZX1o~K0p-5lT3F7yZ4bC8dE2jX9aQ6nO2rP3uS7wT5mV8jW1oY6xB3sD09tR4vU3qM1nG7kL6hV5wY2pJ0aF3eK9dQ8xN4mS2zB7oU5tL1cJ3vX6yP8rE2wO9n"
  }

Token request parameter added by the server when requesting a token with PKCE protection:

• code_verifier=CODE_VERIFIER

[banterCZ]

See this blog post https://sultanov.dev/blog/authorization-code-flow-with-pkce-in-spring-security-oauth/