Test CLI startup time with CI/CD job

[juhoinkinen]

Adds a GitHub Actions job with a shell script for measuring the CLI startup time to prevent its unnoticed deterioration.

The startup time was optimized in #696, and now it is good make sure the startup remains fast. For example, I noticed that adding type hints (planned in #690) can slow the startup, but this can be avoided by postponing the evaluation of annotations.

The timing script executes annif --help four times and measures user + sys time. If the average of the user + sys time is over a threshold, the job fails. The threshold is set to 0.300 s.

In the five previous runs the measured time was 0.245 s, 0.240 s, 0.252 s, 0.242 s and 0.245 s, but in some earlier runs the measured time was over 0.300 s. So failures can be expected, however I think it does not matter if they are reasonably rare (say ~10% of runs).

Failure of this job does not make the other jobs fail, so unit testing, linting and publishing are run in any case.

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Codecov Report

Patch and project coverage have no change.

Comparison is base (6f13121) 99.66% compared to head (881c270) 99.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #706 +/- ## ======================================= Coverage 99.66% 99.66% ======================================= Files 89 89 Lines 6295 6295 ======================================= Hits 6274 6274 Misses 21 21 ```

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