feat: Enable custom join statements

[darwin67]

resolves #423

Add a new CustomJoin type to allow users to fill in whatever non standard join statement they might use.

In my case of using ClickHouse, we use ARRAY JOIN <nested col> AS <col>, and with this change, it can now be written as

goqu.Select("*").From("test").CustomJoin(goqu.L("ARRAY JOIN nested").As("foo"))

which should result in a query like

SELECT * FROM test ARRAY JOIN nested AS foo

CustomJoin will not add anything, and will take whatever the user passes it, so it's up to the user to make sure if the syntax of the JOIN statement is accurate or not.

[darwin67]

ping: @doug-martin or anyone that can review this.

[iredmail]

I suggest not submitting your own files which are not related to this PR: .envrc, flake.lock, flake.nix, also don't change .gitignore in this PR.

[darwin67]

I suggest not submitting your own files which are not related to this PR: .envrc, flake.lock, flake.nix, also don't change .gitignore in this PR.

I don't mind deleting them, but I use Nix for handling all dependencies related to a project, so I won't have access to any Golang tooling without them.
Also, flake.nix has a quirkiness where the file needs to be within git control to work.

On the flip side, these files will work for anyone using Nix and have everything they need to work on this repo the moment they cd into it.

Regardless, I'd like to get a review or a conditional approval for the PR before removing them. Otherwise I'll need re-add them to follow up for changes, and it's a waste of time.