`TraceQueryInjectComment` etc to support datadog database monitoring propagation

[sgtsquiggs]

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.