add `withSpan` and `withTransaction` API

[trentm]

This issue is to add a apm.withSpan(options, fn) API (and similar for withTransaction) that critically takes a function to which the new RunContext is bound. This would avoid the #2611 footgun that startSpan and startTransaction apply. A possible result of that footgun is shown and discussed at #2929.

Adding this API has been considered since the RunContext work.

A good test of this functionality would be whether it could fully or significantly replace the withApmSpan helper in Kibana (see https://github.com/elastic/kibana/blob/main/x-pack/plugins/apm/server/utils/with_apm_span.ts and https://github.com/elastic/kibana/blob/main/packages/kbn-apm-utils/index.ts).

This apm.withSpan() would be the equivalent of OTel JS's tracer.startActiveSpan(). The OTel tracer.startSpan() is already roughly equivalent to our internal createSpan() -- which was added in the RunContext work to cope with the apm.startSpan() footgun.

Eventually I think these APIs should be preferred and startSpan() and startTransaction() APIs should be deprecated.