Support external custom Formatters

### 🤔 What's the problem you're trying to solve?

I have multiple GoDog testing projects and a custom formatter that i wrote for them. But, i unfortunately found out that a custom formatter must be part of the testing package and compiled with the test files as part of the package, otherwise GoDog fails with an error: "unregistered formatter". I would like to suggest an option for loading a precompiled formatter that is independent of the testing package.
I would like to use this formatter in various different testing projects. This puts me in a situation that i need to copy the source files to each project and recompile the tests. Its harder for me to change my code and follow up the formatter's bugs.

### ✨ What's your proposed solution?

Add an option to load a formatter package. something like:
-f path/to/formatter.bin would be nice. even maybe add a flag for an outside formatter if it would be easier for you to add an option and not adjust the existing one.

### ⛏ Have you considered any alternatives or workarounds?

I'm currently copying the files to each testing project package/directory and recompiling.

The issue does not name files or tests; start by tracing formatter registration and command-line option parsing, especially the proposed -f path. Define how a precompiled formatter can load independently of the test package, then verify that existing in-package formatter behavior remains supported and that the external path is handled.

Written by the indexing model from the issue text.