Validate properties on assignment

[rwb27]

This makes use of the validation logic now added to PropertyInfo to validate properties when they are set from Python. Properties are already validated when set over HTTP.

I've added to existing tests to ensure that we test this in various ways, both by calling __set__ on a mocked Thing and in more realistic contexts.

I have added lt.FEATURE_FLAGS which makes this an opt-in feature. That should mean that we can test against the OFM codebase as-is, and that it should pass when we use a feature branch that enables validation. I can confirm that:

• This branch passes the OFM test suite as-is, i.e. it's backwards compatible (disabled by default). See the test-against-ofm-v3 job.
• The feature can be enabled and pass the OFM test suite with minor modifications, detailed in OFM!555. See the test-against-ofm-feature-branch job.

It's slightly unfortunate that this PR now contains two new things (feature flags and validation) but it didn't make sense to merge one without the other. I would like to improve feature flags further, but I will save that for a future PR.

OFM Feature Branch: v3-labthings-validate-properties-on-set

[barecheck]

Barecheck - Code coverage report

Total: 96.38%

Your code coverage diff: 0.03% ▴

Uncovered files and lines

File	Lines
src/labthings_fastapi/properties.py	505, 803, 807, 830-833, 905, 928, 1183

[bprobert97]

I've got no more comments on the code, but need to investigate the failing test

[rwb27]

@bprobert97 sorry about the failing test, I thought I'd checked that before requesting rereview. It's passing now (and the module hasn't changed, so I'm sure the OFM tests will pass by the morning).