in_forward: Add plugin pause/resume mechanism

[drbugfinder-work]

This PR adds pause functionality for forward input plugin.

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

• [N/A] Example configuration file for the change
• [N/A] Debug log output from testing the change
• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• [N/A] Run local packaging test showing all targets (including any new ones) build.
• [N/A] Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• [N/A] Documentation required for this feature

Backporting

• [N/A] Backport to latest stable release.

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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.

[drbugfinder-work]

root@fbinforward:~/fluent-bit-forward-pause/fluent-bit/build# valgrind ./bin/flb-rt-in_forward 
==3979== Memcheck, a memory error detector
==3979== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3979== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==3979== Command: ./bin/flb-rt-in_forward
==3979== 
Test forward...                                 [ OK ]
Test forward_port...                            [ OK ]
Test tag_prefix...                              [ OK ]
Test unix_path...                               [ OK ]
Test unix_perm...                               [ OK ]
SUCCESS: All unit tests have passed.
==3979== 
==3979== HEAP SUMMARY:
==3979==     in use at exit: 0 bytes in 0 blocks
==3979==   total heap usage: 7,595 allocs, 7,595 frees, 8,414,151 bytes allocated
==3979== 
==3979== All heap blocks were freed -- no leaks are possible
==3979== 
==3979== For lists of detected and suppressed errors, rerun with: -s
==3979== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)