Add a server flag to configure registry URL to be used by Coder dashboard

## Context

Add a server flag `CODER_REGSITRY_URL` that defaults to https://registry.coder.com by default but allows users to configure it to point to their registry in the future. 

> [!NOTE]
> This issue's scope is only to expose it as a configuration variable and is independent of the status of whether we have a deployable registry. 

## Prerequisite

- None