Generate gRPC API using buf

[sevein]

This pull request adds new timestamp fields to Job and Task:

message Job {
	// ...
	google.protobuf.Timestamp start_time = 6;
}

message Task {
	// ...
	google.protobuf.Timestamp start_time = 9;
	google.protobuf.Timestamp end_time = 10;
}

• The API is now described under /proto and the generated can be found at /a3m/api.
• API documents are now managed with buf.build and published to the BSR
• API clients can rely on remote code generation to install generated code using standard packages
• buf can be invoked via make buf ARG=help

Connects to #80.

[sbreker]

Looks great 👍