Support Garmin MFA login and add .fit suffix to temp upload files

[lrybak]

Hello,
While running Fit-File-Faker on a fresh MyWhoosh profile I hit two issues:

1. MFA login support

Garmin(email, password) was constructed without a prompt_mfa callback. When Garmin's SSO required an MFA code, the underlying library raised:

GarminConnectAuthenticationError: MFA Required but no prompt_mfa mechanism supplied

I added a prompt_mfa callback that uses questionary (already a project dependency) to interactively ask the user for the one-time code. After a successful login, tokens are cached as before, so subsequent runs don't re-prompt.

2. Temp files must carry the .fit extension

Fix for:
GarminConnectInvalidFileFormatError: File has no extension: /var/folders/g9/401mxzxx7cj89pmjyvmnw5c00000gn/T/tmp2g2z9cmi

garminconnect validates the file extension in upload_activity() and rejects files without one. I added suffix=".fit" to both NamedTemporaryFile(...) calls in upload_all() and NewFileEventHandler.on_created().

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bdbeb1d) to head (0140a0e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #82   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines         1226      1229    +3     
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+ Hits          1226      1229    +3     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[sgfcz]

good job,
I hit the same bug. Would be great to get this merged/released.