avro schema generator

[lyuboxa]

Description

This PR adds the functionality to generate an Avro schema for each snapshot, create and update operations.
The avro schema is rendered in JSON and added to each record under the postgres.avro.schema metadata key.

The schema may be used to hint about types where precision or other may be lost as they are converted to go types, e.g. dates, numeric, etc.

Usage of the schema is a temporary measure until Conduit schema management is released.

In order for the schema to be emitted, the following config var will need to be set logrepl.withAvroSchema

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[hariso]

@lyuboxa @samirketema I left a few comments, I'm mostly interested in nullability of columns and that we're fine with the list of supported types for now. Awesome work!