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

[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