Updating CryptographyAESKey::encrypt to generate 96 bit IVs for GCM block cipher mode to adhere to JWA RFC

[twwildey]

Updating CryptographyAESKey::encrypt to generate 96 bit IVs for GCM block cipher mode to adhere to the RFC for JWA in jose/backends/cryptography_backend.py

See https://www.rfc-editor.org/rfc/rfc7518.html#section-5.3 for the official RFC requirements for JWA

See panva/jose#678 for related discussion on this issue

Testing done

Ran the following commands, which successfully built the package and passed all unit tests that were not already failing:

# Setup python-jose project
git clone git@github.com:twwildey/python-jose.git

# Install dependencies for python-jose
cd python-jose
python -m venv .venv
source .venv/bin/activate
python3 -m pip install --upgrade build
python3 -m pip install -r requirements.txt
python3 -m pip install -r requirements-dev.txt

# Build project
python -m build

# Run tests for project
python -m pytest

[twwildey]

I am merging this PR, since the unit tests that are failing now are also failing without these changes locally.