flb_encoding: charset encoding for input plugins

[bluebike]

Adds library flb_encoding for doing charset encoding CHARSET => UTF8.

• Uses lib/tutf8e-library.
• Only 8-bit charsets are supported.
• At first in_tail plugin is supported, later in_syslog and others.

---

Enter [N/A] in the box, if an item is not applicable to your change.

Testing Before we can approve your change; please submit the following in a comment:

• [ ] Example configuration file for the change
• [x] Debug log output from testing the change

• [ ] Attached Valgrind output that shows no leaks or memory corruption was found

Documentation

• [x] Documentation required for this feature

---

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.