feat: add wal2json extension

[tunix]

Implements #77

[gbartolini]

Hi @tunix, thanks for your contribution. Would you be so kind to submit again your PR, by following the instructions we have placed in this document? https://github.com/cloudnative-pg/postgres-extensions-containers/blob/main/BUILD.md

Thank you!

[gbartolini]

I have two minor comments regarding this implementation @tunix. First, I recommend waiting for pull request #92 to be merged, as this extension needs to disable the CREATE EXTENSION tests.

After that, you should rescaffold from scratch and edit the files by importing the relevant information you have already prepared.

Second, I suggest adding some custom tests, similar to the PostGIS example, to validate the extension.