Move common pieces of mock REST servers to a new package #1586

Job	Run time
Flutter integration tests (windows)	10m 54s
Build Windows Agent Appx	10m 32s
gRPC files are up to date (agentapi)	1m 47s
gRPC files are up to date (wslserviceapi)	49s
Build wsl-pro-service debian package	3m 27s
Flutter Quality checks (ubuntu, gui/packages/p4w_ms_store)	1m 29s
Flutter Quality checks (ubuntu, gui/packages/ubuntupro)	2m 57s
Flutter Quality checks (windows, gui/packages/p4w_ms_store)	4m 38s
Flutter Quality checks (windows, gui/packages/ubuntupro)	6m 10s
C++ Quality Checks	3m 12s
Go Quality checks (ubuntu, agentapi)	43s
Go Quality checks (ubuntu, contractsapi)	19s
Go Quality checks (ubuntu, mocks)	43s
Go Quality checks (ubuntu, storeapi/go-wrapper/microsoftstore)	23s
Go Quality checks (ubuntu, windows-agent)	48s
Go Quality checks (ubuntu, wsl-pro-service)	51s
Go Quality checks (ubuntu, common)	3m 18s
Go Quality checks (windows, agentapi)	4m 37s
Go Quality checks (windows, contractsapi)	1m 12s
Go Quality checks (windows, mocks)	3m 51s
Go Quality checks (windows, storeapi/go-wrapper/microsoftstore)	1m 24s
Go Quality checks (windows, windows-agent)	3m 39s
Go Quality checks (windows, end-to-end)	3m 12s
Go Quality checks (windows, common)	2m 28s
Run Go tests with mocks (ubuntu, windows-agent)	4m 51s
Run Go tests with mocks (ubuntu, wsl-pro-service)	3m 9s
Run Go tests with mocks (windows, storeapi/go-wrapper/microsoftstore)	5m 24s
Run Go tests with mocks (windows, windows-agent)	3m 22s
Run Go tests with mocks (windows, common)	2m 55s
	1h 33m 4s

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