Stop trimming leading 0s on span ids

[joe-elliott]

Describe the bug
After some discussion with Grafana on this issue:
grafana/grafana#62725

We decided that the best fix is for Tempo to start returning span ids with leading 0s.

[joe-elliott]

Currently we are repurposing TraceIDToHexString() here:
https://github.com/grafana/tempo/blob/main/pkg/traceql/engine.go#L129

To convert a byte slice to a hex string. We should create a new method SpanIDToHexString() that does not strip leading 0s and use it instead. Also search the codebase for any other places we may have used TraceIDToHexString() for span ids and replace with this new method.

SpanIDs are 8 bytes/64 bits of information, but this new method should handle cases where the slice has a shorter length. The output result should always be 16 hex characters long.