Feature gate [ClientVisibilityFilter] in C#

[joshua-spacetime]

Description of Changes

Places the [ClientVisibilityFilter] behind the STDB_UNSTABLE experimental feature flag. Will emit a compiler warning unless the following is added to the module file

#pragma warning disable STDB_UNSTABLE // Enable experimental SpacetimeDB features

API and ABI breaking changes

None

Expected complexity level and risk

0

Testing

The build failed until I enabled STDB_UNSTABLE in the sdk-test-cs module.