Add support for providing a legality callback for dynamic legality in DialectConversion.

This allows for providing specific handling for dynamically legal operations/dialects without overriding the general 'isDynamicallyLegal' hook. This also means that a derived ConversionTarget class need not always be defined when some operations are dynamically legal.

Example usage:

ConversionTarget target(...);
target.addDynamicallyLegalOp<ReturnOp>([](ReturnOp op) {
  return ...
};
target.addDynamicallyLegalDialect<StandardOpsDialect>([](Operation *op) {
  return ...
};

PiperOrigin-RevId: 258884753