feat: Extend the package with unit test coverage functionality

[TStanovcak]

This PR introduces support for generating coverage reports for dbt unit tests using the dbt-coverage package. Until now, dbt-coverage focused exclusively on (data) test coverage. With this enhancement, users can also analyze coverage for dbt unit tests.

The approach should conform the original architecture by adding unit_test option to compute command.

Here's the example of the output:

Coverage report (unit_test)                                                                                                                                                                                                                                                                                                                                                                     
======================================================================                                                                                                                                                                                                                                                                                                                                     
model_name_1                                           0 tests    0.0%                                                                                                                                                                                                                                                                                                                                     
model_name_2                                           8 tests  100.0%                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
======================================================================                                                                                                                                                                                                                                                                                                                                     
Total                                                  8 tests   50.0%  

As model can have more then 1 unit test, report is showing the count of the tests. If there's any, it translates into full percentage coverage.

[sweco]

Hey @TStanovcak, apologies for a later reply. Thank you for the contribution, this is a nice addition to the package. I amended a few things and would like to update your PR and merge it. However, I can't seem to be able to push to your branch.

Can you please check that you fulfil the criteria mentioned here? Most commonly the Allow edits from maintainers is disabled on the PR (docs).

[sweco]

Thanks @TStanovcak, I moved the changes to #93 and merged it.