Implement GetDeployment and GetCommand rpcs for external api

[nghialv]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #1147

Does this PR introduce a user-facing change?:

NONE

[pipecd-bot]

Code coverage for golang is 33.75%. This pull request decreases coverage by -0.05%.

File	Function	Base	Head	Diff
pkg/app/api/grpcapi/utils.go	getDeployment	--	57.14%	+57.14%
pkg/app/api/grpcapi/utils.go	getCommand	--	0.00%	+0.00%
pkg/app/api/grpcapi/web_api.go	WebAPI.GetCommand	0.00%	0.00%	+0.00%
pkg/app/api/grpcapi/api.go	API.GetDeployment	0.00%	0.00%	+0.00%
pkg/app/api/grpcapi/api.go	API.GetCommand	0.00%	0.00%	+0.00%
pkg/app/api/grpcapi/web_api.go	WebAPI.CancelDeployment	0.00%	0.00%	+0.00%
pkg/app/api/grpcapi/web_api.go	WebAPI.getDeployment	57.14%	--	-57.14%

[khanhtc1202]

/approve

[pipecd-bot]

This pull request is APPROVED by khanhtc1202.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.