FT-778: Add credential list endpoint to the SDKs

[vaibhavatlan]

Made the following changes:

1. Add a method called get_all()
2. Defined a new model called CredentialResponseList
3. Added a new constant called GET_ALL_CREDENTIALS

[Aryamanz29]

Nice progress!

A few minor changes before we proceed with functional testing:

Since there are some checks failing please fix those issues.

You can use the following commands locally before pushing your commit:

1. For code formatting: 🧹

   ./pyatlan-formatter
   

2. For code QA checks (black, flake8, mypy checks): ✅

   ./qa-checks
   

[Aryamanz29]

LGTM 🚀
(I'll perform manual testing locally and then merge this ✅)

Let's now add docs for this feature to Atlan Developer Portal. 📝