feat: DM-4759 model the unlicensed features a ruleset uses

[cph-datamasque]

Adds unlicensed_feature_warnings to Ruleset: one sentence per feature the ruleset uses that the server's licence does not allow.

It is empty when the ruleset needs nothing its licence withholds, and None until the ruleset has passed validation — which is not the same as having no warnings, and the tests pin that distinction.

Requires server version 3.26.18, which is where DM-4759 lands.

[carlosfunk]

datamasque/client/models/ruleset_library.py is not in the diff, so this cannot go on a line.

The server now also returns unlicensed_feature_warnings on libraries.
The ticket asks for the affected SDK types to be updated.
Add the field to RulesetLibrary, and populate it in create_ruleset_library and the update path, as Ruleset does.
Add a test.