feat: connector for Atlassian Crowd

Documentation/connectors/atlassian-crowd.md

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+ Authentication through Atlassian Crowd
+
+## Overview
+
+Atlassian Crowd is a centralized identity management solution providing single sign-on and user identity.
+
+Current connector uses request to [Crowd REST API](https://developer.atlassian.com/server/crowd/json-requests-and-responses/) endpoints:
+* `/user` - to get user-info
+* `/session` - to authenticate the user
+
+Offline Access scope support provided with a new request to user authentication and user info endpoints. 
+
+## Configuration
+To start using the Atlassian Crowd connector, firstly you need to register an application in your Crowd like specified in the [docs](https://confluence.atlassian.com/crowd/adding-an-application-18579591.html).
+
+The following is an example of a configuration for dex `examples/config-dev.yaml`:
+
+```yaml
+connectors:
+- type: atlassian-crowd
+  # Required field for connector id.
+  id: crowd
+  # Required field for connector name.
+  name: Crowd
+  config:
+    # Required field to connect to Crowd.
+    baseURL: https://crowd.example.com/crowd
+    # Credentials can be string literals or pulled from the environment.
+    clientID: $ATLASSIAN_CROWD_APPLICATION_ID
+    clientSecret: $ATLASSIAN_CROWD_CLIENT_SECRET
+    # Optional groups whitelist, communicated through the "groups" scope.
+    # If `groups` is omitted, all of the user's Crowd groups are returned when the groups scope is present.
+    # If `groups` is provided, this acts as a whitelist - only the user's Crowd groups that are in the configured `groups` below will go into the groups claim.  
+    # Conversely, if the user is not in any of the configured `groups`, the user will not be authenticated.
+    groups:
+    - my-group
+    # Prompt for username field.
+    usernamePrompt: Login
+```