Add table schema validator

[dalonsoa]

Adds a table schema validator for the header. This validator checks that, if there is a schema keyword in the header, it follows the standard defined in the Table Schema specification (mostly). This is done via pydantic models, some of them nested, and some of them discriminated based on the field type_, like it is done in PyECN.

This PRs only implements the validation of the schema if present in the header of the file. It does not enforce it or does anything with it. That is a job for future PRs, as it will need to be adapted to each reader/writer.

As the validation.py module was becoming pretty big, I have split it into several submodules within validation package, and the same with the tests. So the changes look more massive than they actually are. Just focus on the following two files:

• csvy/validators/table_schema.py
• tests/validators/tests_table_schema.py

This PR incorporate the changes made in #169, dropping support for python 3.9 and adding support for 3.13. Ignore those changes or review that specific, already merged, PR.

Needless to say, there's no urgency on reviewing this.

Closes #3