Singleton BooleanMySqlParameter (#212)

Motivation:
Enhance resource management and consistency by adopting a singleton
pattern in `BooleanMySqlParameter`.

Modification:
Restructured BooleanMySqlParameter to allow only one instance, ensuring
efficient usage.

Result:
Optimized performance and uniformity in handling MySQL boolean
parameters with a singleton BooleanMySqlParameter.

src/main/java/io/asyncer/r2dbc/mysql/codec/BooleanCodec.java

@@ -48,7 +48,7 @@ public boolean canEncode(Object value) {
 
     @Override
     public MySqlParameter encode(Object value, CodecContext context) {
-        return new BooleanMySqlParameter((Boolean) value);
+        return (Boolean) value? BooleanMySqlParameter.TRUE : BooleanMySqlParameter.FALSE;
     }
 
     @Override
@@ -59,6 +59,10 @@ public boolean canPrimitiveDecode(MySqlColumnMetadata metadata) {
 
     private static final class BooleanMySqlParameter extends AbstractMySqlParameter {
 
+        private static final BooleanMySqlParameter TRUE = new BooleanMySqlParameter(true);
+
+        private static final BooleanMySqlParameter FALSE = new BooleanMySqlParameter(false);
+
         private final boolean value;
 
         private BooleanMySqlParameter(boolean value) {