databroker server backend config (#1127)

* config,docs: add databroker storage backend configuration

* cache: allow configuring which backend storage to use

Currently supported types are "memory", "redis".

docs/configuration/readme.md

@@ -813,6 +813,27 @@ For an example implementation, the in-memory database used by the cache service
 
 - [pkg/databroker/memory](https://github.com/pomerium/pomerium/tree/master/pkg/databroker/memory)
 
+### Data Broker Storage Type
+
+- Environmental Variable: `DATABROKER_STORAGE_TYPE`
+- Config File Key: `databroker_storage_type`
+- Type: `string`
+- Optional
+- Example: `redis`
+- Default: `memory`
+
+The backend storage that databroker server will use, available types: `memory`, `redis`.
+
+### Data Broker Storage Connection String
+
+- Environmental Variable: `DATABROKER_STORAGE_CONNECTION_STRING`
+- Config File Key: `databroker_storage_connection_string`
+- Type: `string`
+- **Required** when storage type is `redis`
+- Example: `":6379"`
+
+The connection string that server will use to connect to storage backend.
+
 ## Policy
 
 - Environmental Variable: `POLICY`