Add link config validation

[therazix]

This PR extracts the Config class from tmt.utils into its own file and also introduces config validation using Pydantic models. Currently, validation is applied only to the link configuration.

Resolves #84.

Pull Request Checklist

• implement the feature
• write the documentation
• extend the test coverage
• update the specification
• adjust plugin docstring
• modify the json schema
• mention the version
• include a release note

[martinhoyer]

Nice!
The '<2' limit for pydantic version won't work as Fedora only have v2.
Do you know why epel9 was never updated to v2?

What do you think about this settings for BaseModel?
https://docs.astral.sh/ruff/settings/#lint_flake8-type-checking_runtime-evaluated-base-classes

[therazix]

I updated Pydantic to v2, but I'm not sure how to make it work on EPEL 9, which only supports v1.

Since Pydantic v2 supports backward compatibility with v1 through pydantic.v1, I downgraded to v1 and added Pydantic to tmt._compat. I'm not sure if this is the best solution, but it should work with both versions and ensure that we only use features from v1.

What do you think about this settings for BaseModel?
https://docs.astral.sh/ruff/settings/#lint_flake8-type-checking_runtime-evaluated-base-classes

It may be a good idea to add that. I didn't encounter any problems without it, but it may be needed in some scenarios. Added in 9481860

[happz]

Looks like key_to_option, https://github.com/teemtee/tmt/blob/main/tmt/utils/__init__.py#L2926, can we re-use the existing function?

[therazix]

Replaced in 15aa64a.

[psss]

As this is a new approach, it would be good to get a few more reviews. Moving to 1.40.

[skycastlelily]

LGTM:)

[skycastlelily]

I followed the extended test ,and put the following ~/.config/link.fmf:

issue-tracker:
  - type: jira
    url: https://issues.redhat.com
    tmt-web-url: http://tmt.testing-farm.io/
    unkown: value

Then run tmt link command, got no warning, I guess we could warn users by printing the str(error.cause),
similar as how your test works? @therazix