Implement `fastapi-mvc update` command

[rszamszur]

• I have searched the issues of this repo and believe that this is not a duplicate.

Feature Request

Implement fastapi-mvc update command for updating the already generated project from the new template version. The command should verify the correctness of fastapi-mvc project and write permissions before calling copier.run_update(). Also, it should handle some standard copier.run_update() exceptions (ex., if the project is not git-tracked). Lastly, if doable, implement an integration test that will verify the update process, for instance:

1. Generate two projects with the same data, one from the old version and another with the newest.
2. Try to update the old project with changes from the new template.
3. Assert both project's checksums for equality (not sure if this will work since there can be some metadata that will invalidate this assumption)

If the above test is doable, test the update process from the first version to the latest. This will ensure update correctness for all the available versions and can indicate the need for migrations in copier-project between specific versions.

[rszamszur]

The idea of comparing checksums for asserting the update process should work:

[rszamszur@draugr:/tmp]$ nix-hash one/update-test/
096f01f74c775c5df70f2d793f2417b2
[rszamszur@draugr:/tmp]$ nix-hash two/update-test/
096f01f74c775c5df70f2d793f2417b2

However, some directories will need to be excluded (ex.: .git, .venv, .idea, etc.). Maybe exclude list should be based on .gitignore?