kernel: Add logging to kernel library C header

Exposing logging in the kernel library allows users to follow
operations. Users of the C header can use
`kernel_logging_connection_create(...)` to pass a callback function to
Bitcoin Core's internal logger. Additionally the level and category can
be globally configured.

By default, the logger buffers messages until
`kernel_loggin_connection_create(...)` is called. If the user does not
want any logging messages, it is recommended that
`kernel_disable_logging()` is called, which permanently disables the
logging and any buffering of messages.

Co-authored-by: stringintech <stringintech@gmail.com>

src/logging.cpp

@@ -201,6 +201,7 @@ static const std::map<std::string, BCLog::LogFlags, std::less<>> LOG_CATEGORIES_
     {"txreconciliation", BCLog::TXRECONCILIATION},
     {"scan", BCLog::SCAN},
     {"txpackages", BCLog::TXPACKAGES},
+    {"kernel", BCLog::KERNEL},
 };
 
 static const std::unordered_map<BCLog::LogFlags, std::string> LOG_CATEGORIES_BY_FLAG{