Task: implement functional tests for "import-targets" and merge it in "main"

[MVrachev]

What is the task about?

We have finished all todo items from #188, but we forgot to implement functional tests and merge the changes inside the main branch.
We need those two items to actually complete this feature.

• I agree to follow this project's Code of Conduct

[MVrachev]

How we do bdd tests:

1. add features for the bdd tests
2. discuss the bdd features and agree on them
3. implement the tests themselves

Read more on: https://repository-service-tuf.readthedocs.io/en/stable/devel/development.html

[ivanayov]

Requires repository-service-tuf-cli/#456 to be resolved.