proto/agent: add Delegated Identity API (#8)

Add proto definitions for new Delegated Identity API. This API includes two
rpcs: SubscribeToX509SVIDs and SubscribeToX509Bundles.

The SubscribeToX509SVIDs rpc allows a privileged client to get X509-SVIDs
for a given workload. This method uses a directional gRPC stream.

The request is used to subscribe a workload, and the response (stream
from server to client) is used to send SVID updates (for the subscribed
workload). The client subscription is based on the workload's selectors.
X509-SVIDs for registration entries matching *all* the passed selectors
are returned.

The SubscribeToX509Bundles rpc streams get local and all federated
bundles.

Co-authored-by: Mauricio Vásquez <[email protected]>
Co-authored-by: Raphael Campos <[email protected]>
Co-authored-by: Thiago Navarro <[email protected]>
Signed-off-by: Mauricio Vásquez <[email protected]>
Signed-off-by: Raphael Campos <[email protected]>
Signed-off-by: Thiago Navarro <[email protected]>

Makefile

@@ -24,6 +24,7 @@ protos := \
 
 apiprotos := \
 	proto/spire/api/agent/debug/v1/debug.proto \
+	proto/spire/api/agent/delegatedidentity/v1/delegatedidentity.proto \
 	proto/spire/api/server/agent/v1/agent.proto \
 	proto/spire/api/server/bundle/v1/bundle.proto \
 	proto/spire/api/server/debug/v1/debug.proto \