New validation API

[dweindl]

For petab.v2, refactor the current validation setup to

• be more modular
• allow PEtab extensions to register additional validation steps in the future
• make validation messages accessible programmatically (previously, they were logged directly)

Also:

• Make petablint work with both v1 and v2 problems.
• Add first validation rules that are specific to v2

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.08602% with 156 lines in your changes missing coverage. Please review.

Project coverage is 72.92%. Comparing base (6fff855) to head (3ad773d).

Files	Patch %	Lines
petab/v2/lint.py	57.21%	63 Missing and 20 partials :warning:
petab/petablint.py	0.00%	38 Missing :warning:
petab/versions.py	0.00%	20 Missing :warning:
petab/v2/problem.py	44.44%	12 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #288 +/- ## =========================================== - Coverage 74.37% 72.92% -1.45% =========================================== Files 70 72 +2 Lines 4460 4717 +257 Branches 953 1012 +59 =========================================== + Hits 3317 3440 +123 - Misses 872 983 +111 - Partials 271 294 +23 ```

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