Improving the tracing of the declarative schema changer

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

Currently, when a declarative schema changer is running, we capture trace data under a single span called 'NEW SCHEMA CHANGE.' It would be better to have more granular trace support so that we can identify when the different phases start and end.

**Describe the solution you'd like**
We can do two things:
- Create a new span when we wait in waitForOneVersion and waitForNoVersion. Since these functions can be time-consuming, it would be helpful to see their occurrences in the Jaeger output.
- Create a new span for each stage of the schema change. This will allow us to clearly see what stage we are in when inspecting the trace of a schema change that is still running.

Jira issue: CRDB-39832

Epic CRDB-42485