cfl_record_accessor: cfl_ra_key: Implement generic CFL based record accessor

[cosmo0920]

Currently, we didn't implement CFL based record accessor. Which is also called as cobj.
This leads that we're not able to access nested cobjets in processor plugins.
To resolve this, I implemented cobj based record accessor and that infrastructure, cobj_ra_key.

In cobj based record accessor, the same mechanism of parser for record accessor syntax is able to use.
So, I didn't implement cobj specific parser of that syntax.
The main difference of the accessor is creating cobj based record accessor related sturct and use it there.
Other part is mostly copy and paste and then adjust for the operation of cobj.

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

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

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

The written down test cases are not causing memory leaks:

$ valgrind bin/flb-it-cobj_record_accessor -v
<snip>
==878874== 
==878874== HEAP SUMMARY:
==878874==     in use at exit: 0 bytes in 0 blocks
==878874==   total heap usage: 16,823 allocs, 16,823 frees, 1,681,429 bytes allocated
==878874== 
==878874== All heap blocks were freed -- no leaks are possible
==878874== 
==878874== For lists of detected and suppressed errors, rerun with: -s
==878874== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

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

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Backporting

• Backport to latest stable release.

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[edsiper]

Due to the use cases we have, where record accessor patterns will be used go beyond logs, it will apply to metrics and traces, for hence I think the following changes are necessary:

• cobj is only about logs, however record accessor internally operates on top of a CFL data structure. cobj is just a wrapper
• we need to have record accessor API for other signal types such as metrics and traces

The change proposed, is to have a flb_cfl_record_accessor(struct cfl_variant *var) or flb_cfl_record_accessor(struct cfl_kvlist *kvist) so it can be used for any component inside Fluent Bit.