Add endpoint to distributor to push tempopb.Trace

[yvrhdn]

What this PR does:

Adds a new endpoint to the distributor that accepts tempopb.Trace directly.

This makes it possible to push tempopb directly without passing through the OTel Collector receiver. It avoids work associated with converting ptrace.Traces to tempopb.Trace.

I have also extended tempopb/trace_utils.go with handy functions to count spans in a trace and to convert tempopb to OTLP and back.

Which issue(s) this PR fixes:

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yvrhdn]

I think the idea of adding an endpoint for pushing tempopb directly is fine, but if we have otlp converting it to tempopb to send it ends up being the same work, just in a different place.