Setup SQL server based on pgx frontend/backend protocol

[k-anshul]

closes #4817

[begelundmuller]

Two overall comments:

1. It's worth going over the PR and refactoring or clarifying use of the word "Postgres". There are many places where it's not clear that the code pertains to Postgres wire compatibility, as opposed to connecting to an external Postgres database. In some cases, a docstring would help, in other cases, it's worth considering using a term like psql or pgwire instead of postgres.
2. The code in runtime/resolvers/postgres_sql.go is pretty hard to navigate, despite the logic being fundamentally pretty simple. I suggest taking a fresh look at the code with a focus on simplifying and documenting the code. There's also a question (raised in one of the below comments) of whether this should even be a resolver, or if it's better implemented in a dedicated package for Postgres wire compatibility.

[begelundmuller]

Could we use the math/big package directly and avoid this dependency?

[k-anshul]

My understanding is that we need shopspring/decimal to easily encode Numeric : [https://github.com/jackc/pgx/issues/56

[begelundmuller]

Didn't go in depth here, but shopstring/decimal is not a dependency of the pgx project (based on its go.mod file), so they must internally have found a different way around that?

[k-anshul]

Sorry I posted the wrong link.pgx recommends using shopspring/decimal : https://github.com/jackc/pgx/wiki/Numeric-and-decimal-support
The default solution can only be used for float64 or string.