tools+.: add custom linter configuration file

Add this file both to the main LND directory so that devs can use it for
local linter runs and also add it to the `tools` directory so that the
docker environment used to run the linter in CI has access to it. A
custom linter binary can be built via `golangci-lint custom`. This will
pull in and register all the plugins listed in the new config file when
building the new binary. The new binary can then be run using
`custom-gcl run`.

Makefile

@@ -319,7 +319,7 @@ check-go-version: check-go-version-dockerfile check-go-version-yaml
 #? lint-source: Run static code analysis
 lint-source: docker-tools
 	@$(call print, "Linting source.")
-	$(DOCKER_TOOLS) golangci-lint run -v $(LINT_WORKERS)
+	$(DOCKER_TOOLS) custom-gcl run -v $(LINT_WORKERS)
 
 #? lint: Run static code analysis
 lint: check-go-version lint-source