Expose Synapse's `msc4306_enabled` experimental flag as a first-class MDAD
variable and wire it into `homeserver.yaml` alongside the other experimental
feature toggles.

This makes thread-subscriptions support explicit in playbook configuration,
rather than requiring operators to inject the upstream flag via raw
`matrix_synapse_configuration_extension_yaml`.

The variable intentionally controls only the Synapse feature flag. It does not
change the default `thread_subscriptions` worker count, which remains `0` in the
standard presets. Keeping those as separate choices avoids auto-starting an
experimental worker just because the upstream feature toggle is enabled.

Refs:
- b99a58719b/synapse/config/experimental.py (L600-L602)
- b99a58719b/synapse/rest/client/versions.py (L183-L184)