Update InternalAgentLogger with option to write to file

[littleaj]

This will make this a more consistent experience when configuring the SDK and the agent. SDK allows logging to CONSOLE or FILE, but the agent only prints to stdout.

Solution
Add the functionality to the agent logger, sharing as much code as possible with core.

[dhaval24]

@littleaj and @grlima I would rather propose SLF4J as a permanent solution. @littleaj if you wish, I can experiment on SLF4J in parallel, as I am sure it would be a longer fix and we need this to have rather sooner. So I am fine, if we get the internal logger file implementation for now.

[grlima]

@dhaval24 - SLF4J makes sense to me as well. Let's consolidate all SDK logging around SLF4J.

[dhaval24]

@grlima great! Let's do that. @littleaj as we discussed, my understanding is that you wanna take a stab at this. Correct? Please feel free to take over if you would like, or alternatively let me know. I will plan some evenings / weekends for it if needed.

[dhaval24]

@littleaj I have renamed this, as we all are on same page for SLF4J work. I am also linking the past feature request. #309