internal/database: add DB.WithPGXConn
internal/database: add DB.WithPGXConn

This function lets code outside the package use pgx-specific features
like CopyFrom.

Change-Id: I40d2c8b5d9a92f61f535fc25059fb976baf5d178
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/307870
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Run-TryBot: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite