feat: OAuth2: Implement OAuth 2.0 Implicit Grant

[pietrygamat]

Implements OAuth2 Implicit Grant authorization

Description

Contribution Checklist:

• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
• I have added screenshots or gifs to help explain the change if applicable.
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

resolves #2056
resolves #1901

[pietrygamat]

FYI, the mockup is from

meta {
  name: implicit
  type: http
  seq: 6
}

get {
  url: http://localhost:8080/realms/bruno/protocol/openid-connect/userinfo
  body: none
  auth: oauth2
}

auth:oauth2 {
  grant_type: implicit
  callback_url: bruno://auth
  authorization_url: http://localhost:8080/realms/bruno/protocol/openid-connect/auth?login_hint=bruno-user
  client_id: brunoclient
  scope: openid
}

against locally started Keycloak.
For reproducable environment, place the following realm-export.json file in an empty directory and start Keycloak in Docker:

docker run -p 8080:8080 \
-e KEYCLOAK_ADMIN=admin \
-e KEYCLOAK_ADMIN_PASSWORD=admin \
-v $(pwd):/opt/keycloak/data/import:z quay.io/keycloak/keycloak:24.0.1 \
start-dev --import-realm

realm-export.json

[pietrygamat]

While functional on its own, this may be superseded by #2077 where more complete overhaul of oauth2 is proposed, resulting in a more consistent user experience between different oauth2 grants.