fix(spans): Infer names PII-safely

[loewenheim]

When inferring names for V1 spans, PII has to be taken into account. This means that unconditionally inferring names during V1 -> V2 conversion, as we do now, is unsafe—it's fine for spans that have gone through PII scrubbing (to wit, standalone spans that have gone through the legacy pipeline, as well as spans extracted from transactions), but for standalone spans we have to do it later.

This addresses the problem in the following ways:

• It adds a flag to span_v1_to_span_v2 for whether to infer a name.
• It adds an infer_name flag to span processing settings, controlling whether names should be inferred in the pipeline. This is disabled for V2 spans but enabled for legacy spans.

With this PR, name inference is now PII-safe in all cases:

• transaction spans go through the transaction pipeline, are scrubbed, and have the name inferred at the very end during conversion;
• the same for standalone spans in the legacy pipeline;
• standalone spans in the V2 pipeline go through V2 scrubbing and have their name inferred after that;
• V2 spans never have a name inferred.

Closes INGEST-957.

[linear-code]

INGEST-957