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`.

tools/.custom-gcl.yml

@@ -0,0 +1,4 @@
+version: v1.57.0
+plugins:
+  - module: 'github.com/lightningnetwork/lnd/tools/linters'
+    path: ./linters