Allow types to specify preference format result and param formats

This will be useful for array and composite types that may have to
support elements that may not support binary encoding.

It also is slightly more convenient for text-ish types to have a default
format of text.

pgtype.go

@@ -142,6 +142,18 @@ type TypeValue interface {
 	TypeName() string
 }
 
+// ResultFormatPreferrer allows a type to specify its preferred result format instead of it being inferred from
+// whether it is also a BinaryDecoder.
+type ResultFormatPreferrer interface {
+	PreferredResultFormat() int16
+}
+
+// ParamFormatPreferrer allows a type to specify its preferred param format instead of it being inferred from
+// whether it is also a BinaryEncoder.
+type ParamFormatPreferrer interface {
+	PreferredParamFormat() int16
+}
+
 type BinaryDecoder interface {
 	// DecodeBinary decodes src into BinaryDecoder. If src is nil then the
 	// original SQL value is NULL. BinaryDecoder takes ownership of src. The
@@ -364,7 +376,9 @@ func (ci *ConnInfo) RegisterDataType(t DataType) {
 
 	{
 		var formatCode int16
-		if _, ok := t.Value.(BinaryEncoder); ok {
+		if pfp, ok := t.Value.(ParamFormatPreferrer); ok {
+			formatCode = pfp.PreferredParamFormat()
+		} else if _, ok := t.Value.(BinaryEncoder); ok {
 			formatCode = BinaryFormatCode
 		}
 		ci.oidToParamFormatCode[t.OID] = formatCode
@@ -372,7 +386,9 @@ func (ci *ConnInfo) RegisterDataType(t DataType) {
 
 	{
 		var formatCode int16
-		if _, ok := t.Value.(BinaryDecoder); ok {
+		if rfp, ok := t.Value.(ResultFormatPreferrer); ok {
+			formatCode = rfp.PreferredResultFormat()
+		} else if _, ok := t.Value.(BinaryDecoder); ok {
 			formatCode = BinaryFormatCode
 		}
 		ci.oidToResultFormatCode[t.OID] = formatCode