connector: implement Microsoft connector

connector/microsoft implements authorization strategy via Microsoft's
OAuth2 endpoint + Graph API. It allows to choose what kind of tenants
are allowed to authenticate in Dex via Microsoft:
  * common - both personal and business/school accounts
  * organizations - only business/school accounts
  * consumers - only personal accounts
  * <tenant uuid> - only account of specific tenant

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

Documentation/microsoft-connector.md

@@ -0,0 +1,66 @@
+# Authentication through Microsoft
+
+## Overview
+
+One of the login options for dex uses the Microsoft OAuth2 flow to identify the
+end user through their Microsoft account.
+
+When a client redeems a refresh token through dex, dex will re-query Microsoft
+to update user information in the ID Token. To do this, __dex stores a readonly
+Microsoft access and refresh tokens in its backing datastore.__ Users that
+reject dex's access through Microsoft will also revoke all dex clients which
+authenticated them through Microsoft.
+
+## Configuration
+
+Register a new application on https://apps.dev.microsoft.com via `Add an app`
+ensuring the callback URL is `(dex issuer)/callback`. For example if dex
+is listening at the non-root path `https://auth.example.com/dex` the callback
+would be `https://auth.example.com/dex/callback`.
+
+The following is an example of a configuration for `examples/config-dev.yaml`:
+
+```yaml
+connectors:
+  - type: microsoft
+    # Required field for connector id.
+    id: microsoft
+    # Required field for connector name.
+    name: Microsoft
+    config:
+      # Credentials can be string literals or pulled from the environment.
+      clientID: $MICROSOFT_APPLICATION_ID
+      clientSecret: $MICROSOFT_CLIENT_SECRET
+      redirectURI: http://127.0.0.1:5556/dex/callback
+```
+
+`tenant` configuration parameter controls what kinds of accounts may be
+authenticated in dex. By default, all types of Microsoft accounts (consumers
+and organizations) can authenticate in dex via Microsoft. To change this, set
+the `tenant` parameter to one of the following:
+
+- `common`- both personal and business/school accounts can authenticate in dex
+  via Microsoft (default)
+- `consumers` - only personal accounts can authenticate in dex
+- `organizations` - only business/school accounts can authenticate in dex
+- `<tenant uuid>` or `<tenant name>` - only accounts belonging to specific
+  tenant identified by either `<tenant uuid>` or `<tenant name>` can
+  authenticate in dex
+
+For example, the following snippet configures dex to only allow business/school
+accounts:
+
+```yaml
+connectors:
+  - type: microsoft
+    # Required field for connector id.
+    id: microsoft
+    # Required field for connector name.
+    name: Microsoft
+    config:
+      # Credentials can be string literals or pulled from the environment.
+      clientID: $MICROSOFT_APPLICATION_ID
+      clientSecret: $MICROSOFT_CLIENT_SECRET
+      redirectURI: http://127.0.0.1:5556/dex/callback
+      tenant: organizations
+```