core: added filesystem abstraction layer

[leonardo-albertovich]

This PR introduces a platform agnostic filesystem interface that implements functionality that used to be repeated in multiple plugins.

A platform agnostic change monitoring component based on native facilities (inotify, windows fs change notifications, FSEvent, etc) will be added before this is merged.

This PR does not change any of the existing code in significative ways other than renaming flb_file_read to flb_file_read_contents.

Note: This PR contains the comoponents introduced by PR 7564 in a way that can be merged immediately with no compromises and another PR will be opened to update in_tail to use the new abstraction layer.

[edsiper]

@leonardo-albertovich is this one is ready pls remove the draft mode

[leonardo-albertovich]

I'd like to get a chance to add the monitoring part before we merge it which I won't be able to work on for about two weeks so I think it'd be better to leave it as a draft for the moment.