[eliminate-warden#78779164]: Change submodule protobuf to garden-protocol at master.

Signed-off-by: Glyn Normington <gnormington@pivotal.io>
[eliminate-warden#78779164]: Regenerate protocol from new garden-protocol submodule.

Signed-off-by: Glyn Normington <gnormington@pivotal.io>
Rename warden package to api

Change package of types.go to garden.

Signed-off-by: Steve Powell <spowell@pivotal.io>
Rename Warden* to Garden*

This includes WardenError and WardenServer types which may be used by
clients.

Signed-off-by: Steve Powell <spowell@pivotal.io>

[#78779164]
Merge branch 'eliminate-warden#78779164'