add uuid and tag registers

[Poofjunior]

R_UUID and R_TAG are now implemented.

If PICO_RP2040 is defined (default for the pico-sdk), then the lower 8 bytes (bytes 8-15) will be populated with the 8-byte sequence from the QSPI flash upon instantiation of a HarpCore. This 64-bit value is unique per flash chip instance, making it a good default value to stuff into this register.

For the R_TAG register, an 8 byte sequence must be added into the constructor.
For folks using CMake (most-likely true if you're using this project), you can stuff it with a string representation of the 7-byte short Git hash by adding the following to your CMakeLists.txt:

find_package(Git REQUIRED)
execute_process(COMMAND "${GIT_EXECUTABLE}" rev-parse --short HEAD OUTPUT_VARIABLE COMMIT_ID OUTPUT_STRIP_TRAILING_WHITESPACE)
message(STATUS "Computed Git Hash: ${COMMIT_ID}")
add_definitions(-DGIT_HASH="${COMMIT_ID}") # Usable in source code.

After that, you can simply use GIT_HASH in your source code.

References:

Implements harp-tech/protocol#38 and harp-tech/protocol#39

[Poofjunior]

Worth considering: should we remove the tag from the constructor and just expose it through a setter function?