PG-857: use common JSON API for keyring

[dAdAbird]

This commit replaces dependency of the keyring code on JSON backend functions with common JSON API.

Usage of the backend JSON funcs prevents the code to be used by frontend tools like pg_waldump. Besides it requiers extra conversions to Datums and DirectFunctionCall-s.

For: https://perconadev.atlassian.net/browse/PG-857

[dutow]

Looks working, and okay, but...

My main issue with this pull request is that now instead of a nice, simple and easy to understand JSON code we end up with a long and complex thing that works, but it is difficult to understand and/or modify.

I already had this issue with the previous JSON refactoring where we removed jsonc, because that already made the code more complex, but now it is even more so.

[dutow]

Why? I think the issue here is that "file" remote support is still missing from the code, plese read the documentation: https://percona-lab.github.io/pg_tde/main/external-parameters.html

[dAdAbird]

Remote is supported but with URL only and not path. It didn't make sense to me at first and I thought this is some leftovers (was about to clarify in it).
I'll do the "remote path" but the question is should the 'type' parameter be like in tests file or like in docs remote?

test L33:

json_object( 'type' VALUE 'file', 'path' VALUE '/tmp/datafile-location' )

docs:

json_object( 'type' VALUE 'remote', 'path' VALUE '/tmp/datafile-location' )

[dAdAbird]

To the comment of complexity - I agree, but we don't have much choice for this code to be used with frontend tools. The other way would be get back to json-c, but it's an external dependency...
I'll add more comments on parser and examples of possible json inputs