Request: make lttng-ust an optional build dependency

[tmds]

Other than .NET, RHEL doesn't have packages that require lttng-ust as a build-time dependency.
To build .NET for RHEL, the .NET package maintainers have been building lttng packages as well for the past .NET releases.

This is a request to make lttng-ust an optional build dependency.

cc @jkotas @omajid @MichaelSimons

[dotnet-policy-service]

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.

[jkotas]

LTTNG support is under FEATURE_EVENTSOURCE_XPLAT, so this should be about exposing it as the top-level build option and making sure that it works.