Merge changes from topic "tokio-test-utils"

* changes:
  [Private GATT] Simplify datastore interface
  [Private GATT] Add support for MTU Exchange
  [Private GATT] Add support for descriptors
  [Private GATT] Add support for handle value indications
  [Private GATT] Make ATT permissions a bitmask
  [Private GATT] Refactor AttOpcode handling
  [Private GATT] Rename AttTransactionHandler
  [Private GATT] Refactor to remove Rc<>
  [Private GATT] Add support for Tokio test-utils