feat: implement model constraint export

[tlm]

This commit introduced a new model export operation for setting the model's constraints on export. The decision was made to seperate the export operations of the model domain into seperate entities so they can be easily tested.

This PR is a bit wishful at the moment as we don't have a reasonable test plan that can assert this work's besides the unit tests included. To merge this PR we need to:

• Merge feature: introduce model constraints importing #18760 import operation for model constraints
• Merge feat(dqlite): use model service for getting/setting model constraints in facade layer #18754 moving facades over to using model constraints service.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• [ ] Integration tests, with comments saying what you're testing
• [ ] doc.go added or updated in changed packages

QA steps

Test 1

Confirm the unit tests in this PR work

Test 2

This test will not work until the above mentioned items have landed.

1. Bootstrap two controllers (one, two). Add a model to controller one called foo.
2. On model foo set the model constraint for memory to 1024M
3. Deploy the ubuntu charm to model foo and confirm everything is active in the model status.
4. Migrate the model foo from controller one to controller two
5. On controller two run juju model-constraints on the migrated model and confirm the memory constraint is set and applied.

Documentation changes

N/A

Links

Jira card: JUJU-7263