Make MO_CONFIG_MAX_VALSTRSIZE override-able via -D

Wrap the existing 128-byte default in #ifndef so consumers can raise the
ceiling via a build flag (`-DMO_CONFIG_MAX_VALSTRSIZE=256` etc.) without
patching the header. Default is unchanged. Same shape as the surrounding
MO_CONFIG_EXT_PREFIX / MO_CONFIG_TYPECHECK guards.

Motivation: the OCPP 1.6 MeterValuesSampledData advertised list overflows
128 chars once the optional measurands (Temperature, SoC, Frequency,
Power.Factor) are bound — Configuration::setString silently rejects the
write and the CSMS-side default permanently overrides ours.

Author: RAR

src/MicroOcpp/Core/ConfigurationKeyValue.h

@@ -8,7 +8,9 @@
 #include <ArduinoJson.h>
 #include <memory>
 
+#ifndef MO_CONFIG_MAX_VALSTRSIZE
 #define MO_CONFIG_MAX_VALSTRSIZE 128
+#endif
 
 #ifndef MO_CONFIG_EXT_PREFIX
 #define MO_CONFIG_EXT_PREFIX "Cst_"