Add TxOptions.BeginQuery to allow overriding the default BEGIN query

https://github.com/jackc/pgx/issues/1643

tx.go

@@ -44,6 +44,10 @@ type TxOptions struct {
 	IsoLevel       TxIsoLevel
 	AccessMode     TxAccessMode
 	DeferrableMode TxDeferrableMode
+
+	// BeginQuery is the SQL query that will be executed to begin the transaction. This allows using non-standard syntax
+	// such as BEGIN PRIORITY HIGH with CockroachDB. If set this will override the other settings.
+	BeginQuery string
 }
 
 var emptyTxOptions TxOptions
@@ -53,6 +57,10 @@ func (txOptions TxOptions) beginSQL() string {
 		return "begin"
 	}
 
+	if txOptions.BeginQuery != "" {
+		return txOptions.BeginQuery
+	}
+
 	var buf strings.Builder
 	buf.Grow(64) // 64 - maximum length of string with available options
 	buf.WriteString("begin")