Add record accessor support in parser filter

[agup006]

Summary

• allow Parser filter to parse nested keys using record accessor patterns
• clean up parser context on exit
• add unit test covering record accessor pattern

Testing

• cmake -DFLB_DEV=On -DFLB_TESTS_RUNTIME=On -DFLB_TESTS_INTERNAL=On ..
• make -j$(nproc)
• ./bin/flb-rt-filter_parser

[agup006]

SUCCESS: All unit tests have passed.
==93006==
==93006== HEAP SUMMARY:
==93006== in use at exit: 0 bytes in 0 blocks
==93006== total heap usage: 30,489 allocs, 30,489 frees, 14,191,579 bytes allocated
==93006==
==93006== All heap blocks were freed -- no leaks are possible
==93006==
==93006== For lists of detected and suppressed errors, rerun with: -s
==93006== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)