feat: estimated completion date

[sprestrelski]

Closes #3. Adds function that calculates estimated completion date based on annotations made in the past N days (default=1).

Sample outputs from today

Total: 6178
Estimated time to completion: 5.3 days
1247 annotations were made in the last 24 hours

Total: 6178
Estimated time to completion: 12.5 days
3702 annotations were made in the last 168 hours

[sprestrelski]

i'm fixing pylint!!

[ntlhui]

************* Module label_studio_slack_reporter.main
label_studio_slack_reporter/main.py:47:74: C0303: Trailing whitespace (trailing-whitespace)
label_studio_slack_reporter/main.py:48:77: C0303: Trailing whitespace (trailing-whitespace)
label_studio_slack_reporter/main.py:82:0: C0303: Trailing whitespace (trailing-whitespace)
label_studio_slack_reporter/main.py:84:0: C0303: Trailing whitespace (trailing-whitespace)
label_studio_slack_reporter/main.py:16:0: R0914: Too many local variables (18/15) (too-many-locals)
label_studio_slack_reporter/main.py:86:0: R09[13](https://github.com/UCSD-E4E/label-studio-slack-reporter/actions/runs/11863527138/job/33065150929?pr=4#step:5:14): Too many arguments (6/5) (too-many-arguments)
label_studio_slack_reporter/main.py:86:0: R0917: Too many positional arguments (6/5) (too-many-positional-arguments)

Please resolve whitespace.

You can add pylint: disable= bypass for the other ones.