Enable service.profilesSupport feature gate so profiles pipelines can start #2273

Is your feature request related to a problem? Please describe.

Configuring a profiles pipeline makes the agent fail to start:

pipeline "profiles": profiling signal support is at alpha level, gated under the "service.profilesSupport" feature gate

Profiles are gated behind service.profilesSupport, which is alpha and off by
default in the collector the agent embeds. The agent builds the collector's
arguments itself and only passes --config=, so there is no supported way for an
operator to enable the gate. Profiles are unreachable from the agent today.

Describe the solution you'd like

Have the agent enable service.profilesSupport by default when it starts the
collector, so profiles pipelines work without operators needing to know the gate
exists.

Describe alternatives you've considered

Exposing a general --feature-gates passthrough in the agent config, or an
environment-variable opt-in. Both leave profiles off by default and require
operators to know which gate to name.

Additional context

I have a change ready and would like to submit a PR for this.