Library failing when registering drivers in parallel on different goroutines.

Internal `driverRegistry` wasn't goroutine safe, causing race conditions with
multiple writes in the internal maps.

```
$ go test -race -run TestRegister

WARNING: DATA RACE
Write at 0x00c000254000 by goroutine 9:
  runtime.mapaccessK()
      /opt/homebrew/Cellar/go/1.19/libexec/src/runtime/map.go:518 +0x1ec
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.(*driverRegistry).add()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql.go:60 +0x144
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.TestRegister.func1()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql_test.go:282 +0x54
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.TestRegister.func2()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql_test.go:285 +0x44

Previous write at 0x00c000254000 by goroutine 15:
  runtime.mapaccessK()
      /opt/homebrew/Cellar/go/1.19/libexec/src/runtime/map.go:518 +0x1ec
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.(*driverRegistry).add()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql.go:60 +0x144
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.TestRegister.func1()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql_test.go:282 +0x54
  gopkg.in/DataDog/dd-trace-go.v1/contrib/database/sql.TestRegister.func2()
      /Users/rafael.justo/dev/go/src/github.com/rafaeljusto/dd-trace-go/contrib/database/sql/sql_test.go:285 +0x44
```