`TraceQueryInjectComment` etc to support datadog database monitoring propagation

**Is your feature request related to a problem? Please describe.**
DataDog propagates tracing to the database via sql comments. There is no way to do this w/ the current tracing ifaces.

**Describe the solution you'd like**
Add additional ifaces specifically for this feature that would allow injecting a comment infront of all sql sent to the db.

**Describe alternatives you've considered**
CopyFrom, Commit, etc are hard-coded sql queries that cannot be modified without editing pgx source.
You could get _most_ of the rest working using the database/sql pgx iface.

If this feature wouldn't be outright rejected, I'm happy to submit a PR.

Start by reviewing the tracing interfaces and the proposed TraceQueryInjectComment capability, then compare the CopyFrom and Commit paths with the database/sql pgx interface. Done means defining interfaces that allow Datadog SQL comments to be injected before all SQL sent to PostgreSQL, including the currently hard-coded queries.

Written by the indexing model from the issue text.