Expose control API for builds, syncs, and deploys

[nkubala]

this change introduces an API for controlling the individual components (build, sync, and deploy) of Skaffold. this API can be accessed through the gRPC/HTTP servers that skaffold is already starting up when in dev mode.

each component can be set to run in "api mode" through its own flag (--build-trigger=api, --deploy-trigger=api, --sync-trigger=api) leaving the others to run as normal. optionally, the --api-mode=true flag can be set, which implicitly sets all triggers to api. this flag serves only to eliminate verbosity on the command line.

when in "api mode", a component's action will not be performed until a user sends an "intent" to the skaffold server. multiple intents can be sent at one time, for example a "build" and "deploy" can be sent simultaneously. the ordering of these actions is still preserved in the dev loop logic. changes to the local filesystem are still computed as tracked as normal; skaffold "remembers" them until the corresponding user intent is received.

examples of user intent being sent to skaffold:

RPC

conn, _ = grpc.Dial(fmt.Sprintf(":%s", skaffoldRPCPort), grpc.WithInsecure())
client = proto.NewSkaffoldServiceClient(conn)
client.Execute(context.Background(), &proto.UserIntentRequest{
    Intent: &proto.Intent{
          Build: true,
          Deploy: true,
    },
})

HTTP
curl http://localhost:<skaffold_http_port>/v1/execute -d '{"build": true, "deploy": true}'

Fixes #1556
Fixes #1520
Fixes #1396

[codecov]

Codecov Report

Merging #2450 into master will increase coverage by 0.01%.
The diff coverage is 65.53%.

Impacted Files	Coverage Δ
pkg/skaffold/config/options.go	100% <ø> (ø)	⬆️
pkg/skaffold/trigger/triggers.go	30.95% <ø> (+1.82%)	⬆️
pkg/skaffold/runner/listen.go	0% <0%> (ø)	⬆️
pkg/skaffold/server/endpoints.go	0% <0%> (ø)	⬆️
cmd/skaffold/app/cmd/dev.go	86.66% <100%> (+2.05%)	⬆️
pkg/skaffold/runner/intent.go	100% <100%> (ø)
pkg/skaffold/server/server.go	58.57% <35.71%> (-7.54%)	⬇️
pkg/skaffold/runner/runner.go	69.56% <70.58%> (-0.73%)	⬇️
pkg/skaffold/runner/changeset.go	84% <77.77%> (-16%)	⬇️
pkg/skaffold/runner/dev.go	66.32% <81.48%> (+2.69%)	⬆️
... and 3 more

[balopat]

Other than the last couple of comments I think this is looking pretty good actually 👍

[nkubala]

unfortunately this design isn't working with the notify trigger as is. once fsnotify tells skaffold something has changed, it kicks off a dev loop, but if skaffold hasn't received a user intent to build, the dev loop will exit...and won't try again until fsnotify detects a new change :( working on a fix for this now.

update: this should be working with the notify trigger now. added an intent channel that the server sends back on and is subscribed to by the listener, as an alternative entrypoint into the dev loop

[balopat]

LGTM!! @dgageot ?

[dgageot]

I would merge after the release of 0.34 to give us time to test well