Detect custom AppSignal client files from diagnose command

[luismiramirez]

From a leftover of #882

Even tho we don't recommend it, some users might initialize the AppSignal client with other methods instead of the recommended appsignal.cjs. If that happens, the diagnose command won't be able to detect the client initialization file.

We tried to use the --config flag, but with the current implementation, that means we need to share information between the diagnose command and the Config class which is not ideal. In order to make the --config flag to work, more significant changes are required.

ToDo

• Provide users with the ability of telling where the custom AppSignal client initialization file is when using the diagnose command
• Check the presence of this file and load it as we do with appsignal.cjs or src/appsignal.cjs