fix: ensure api/static directory is created by Git

[rolodato]

This prevents a warning starting the API if this directory does not exist:

    /Users/rolodato/Library/Caches/pypoetry/virtualenvs/flagsmith-api-Avcq7jRJ-py3.11/lib/python3.11/site-packages/whitenoise/base.py:115: UserWarning: No directory at: /Users/rolodato/source/flagsmith/flagsmith/api/static/
  warnings.warn(f"No directory at: {root}")

Thanks for submitting a PR! Please check the boxes below:

• I have run pre-commit to check linting
• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Adds a .gitkeep file that is allow-listed in api/.gitignore to ensure the api/static directory exists when cloning this repository. This prevents a warning when starting up the API and the directory does not yet exist.

Before:

% make serve
poetry run gunicorn --bind 127.0.0.1:8000 app.wsgi --reload
[2024-04-01 11:02:45 -0300] [46225] [INFO] Starting gunicorn 20.1.0
[2024-04-01 11:02:45 -0300] [46225] [INFO] Listening at: http://127.0.0.1:8000 (46225)
[2024-04-01 11:02:45 -0300] [46225] [INFO] Using worker: sync
[2024-04-01 11:02:45 -0300] [46228] [INFO] Booting worker with pid: 46228
/Users/rolodato/Library/Caches/pypoetry/virtualenvs/flagsmith-api-Avcq7jRJ-py3.11/lib/python3.11/site-packages/whitenoise/base.py:115: UserWarning: No directory at: /Users/rolodato/source/flagsmith/flagsmith/api/static/
  warnings.warn(f"No directory at: {root}")

After:

% make serve
poetry run gunicorn --bind 127.0.0.1:8000 app.wsgi --reload
[2024-04-01 11:08:44 -0300] [46650] [INFO] Starting gunicorn 20.1.0
[2024-04-01 11:08:44 -0300] [46650] [INFO] Listening at: http://127.0.0.1:8000 (46650)
[2024-04-01 11:08:44 -0300] [46650] [INFO] Using worker: sync
[2024-04-01 11:08:44 -0300] [46653] [INFO] Booting worker with pid: 46653

How did you test this code?

Manually by cleaning all untracked files and directories in the repository, and checking that the api/static directory still exists:

% git clean -f -d -x
% file api/static
api/static: directory

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