Add Dictionary class and revert back to sending string as CMDP log payload

Mainly closes #24 (closed) and implements #23 (closed)

This MR does the following things:

• Adds a new Dictionary class to replace the dictionary_t. This class has explicit msgpack pack/unpack functions, so that we do not rely on the less-then-ideal upstream std::variant implementation.
• Replaces the assemble function from Header class with a proper msgpack pack function
• Sends TRACE information in CMDP logging as part of the header as discussed in #23 (closed)
• Adds tests for Header (and Dictionary)

To be done:

• Test dictionary packing with python
• Add test for string output of tags
• Update CMDP (in separate MR)