WIP / core: support configurable event loop mechanism

[edsiper]

This PR makes possible to choose manually the desired Event loop mechanism, otherwise the system does an automatic lookup. The new CMake options available are:

CMake option	Description
FLB_EVENT_LOOP_EPOLL	Linux based event system
FLB_EVENT_LOOP_ POLL	Experimental support for poll(2) based event interface
FLB_EVENT_LOOP_KQUEUE	BSD and Apple based event mechanism
FLB_EVENT_LOOP_SELECT	Old interface, limited to 256 file descriptors only
FLB_EVENT_LOOP_LIBEVENT	System provided by libevent library

Note this PR is still work in process

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[github-actions]

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