Cache Serialize API (#215)

* removed datacatalog ExtendReservation API

Signed-off-by: Daniel Rammer <daniel@union.ai>

* filled out generated content with 'make generate'

Signed-off-by: Daniel Rammer <daniel@union.ai>

* updated protobuf docs for admin and core definitions

Signed-off-by: Daniel Rammer <daniel@union.ai>

* created ReservationID message to reuse

Signed-off-by: Daniel Rammer <daniel@union.ai>

* changed artifact reservation API to not include handling of artifacts - only reservations.

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added cache reservation comments

Signed-off-by: Daniel Rammer <daniel@union.ai>

* fixed indenting issues that always drive me crazy

Signed-off-by: Daniel Rammer <daniel@union.ai>

* updated discovery_reservable to discovery_serializable to adhere to name change

Signed-off-by: Daniel Rammer <daniel@union.ai>

* moved catalog reservation status enum into a message

Signed-off-by: Daniel Rammer <daniel@union.ai>

* changed discovery_serializable TaskMetadata field to cache_serializable

Signed-off-by: Daniel Rammer <daniel@union.ai>