[ENH] estimator dependency management and individual CI

[fkiraly]

This PR adds some tools for estimator dependency management and individual CI.

Used to handle estimators depending on unmaintained packages, or conflicting dependencies, see #625. Depends on #625.

Adds the follwing testing infrastructure:

• a CI step test-est to test individual estimators in their own VM with their own dependency sets
• serialization and dependency extraction utilities deps, imports, and craft
• a tag tests:vm indicating whether tests run on their own VM
• a tag tests:libs that can be used to specify non-inheritance dependencies within skpro

The above are copied and adapted from sktime.

The new tag tests:vm is also applied to the XgboostLSS estimator and the CyclicBoosting estimator, which should run in its own VM if the new test framework works.