Address code review suggestions from 'Introduce QtGrpc' patch

This list may be split into separate stories:

• Add output argument HAS_GRPC/HAS_SERVICES to qt6_add_protobuf() cmake method
• Remove gRPC includes from public QtGrpc headers (qgrpcchannel.h, qgrpcabstractservice.h) 
• Remove QAbstractGrpcClient* client argument from QAbstractGrpcChannel::stream() method (patch)
• Change API of QAbstractGrpcChannel to not duplicate call functionality for sync and async calls, not use return parameter and rename
• Add pkg-config to find gRPC on Debian 
• Use better names for call() and stream() methods in the QAbstractGrpcChannel
• Use return instead of out parameter for QGrpcCallReply* in QAbstractGrpcChannel::call()
• Use return instead of out parameter for QByteArray& in QAbstractGrpcChannel::call(){}
• Find alternative usage that doesn't involve complex QSharedPointer logic
• In any case, don't use QSharedPointer in new code anymore, use std::shared_ptr (2x faster) (patch)
• Rename QGrpcAsyncOperation -> QGrpcOperation (patch)
• Remove QGrpcAbstractService class and disable service generation (patch)
• Rename QAbstractGrpcClient -> QGrpcClientBase (patch)
• explicit all ctors that should be (incl. all except default ctors of QObject subclasses) (patch)
• all polymorphic dtors (incl. pure abstract ones) out-of-line and marked as override (patch)[|https://codereview.qt-project.org/c/qt/qtgrpc/+/439481/comments/6d47c1e3_f04965e3]
• review member functions for const-corretness (patch)
• use = default instead of }} for empty SMFs (patch)
• re-use QObject::d_ptr instead of each class rolling it's own (patch)
• use ""_L1, u""_s instead of QLatin1String/View, QStringLiteral (patch)
• includemocs the whole module (cf. e.g. QTBUG-103207) (patch)
• omit pointless inline keywords (patch)
• use template < consistently, not template< (patch)
• re-think use of QString for methods when the underlying transport only supports 8-bit strings (const char*) (patch)
• use trailing , in enums (patch)
• use only scoped enums or at least a fixed underlying_type (patch)
• don't use defaulted arguments of non-trivial type (QTBUG-98117), overload instead (patch)
• leave empty line after Q_OBJECT, none after fwd declaring Private's (patch)
• no braces around 1-line bodies (patch)
• no empty () on lambdas (patch)
• Don't use 3-parameter QObject::connect; add a context object to run in reasonable threads and define lifetime (patch)
• Use return instead of out parameter for QGrpcStream* in QAbstractGrpcChannel::stream()