[WIP] Add segger RTT as a logger output implementation

[Alloyed]

with this PR, applications can use using Log = daisy::Logger<LOGGER_RTT>; to print their outputs using RTT over the ST-Link debug connection. this is faster than LOGGER_SEMIHOST, and a little easier to set up.

upstream docs: https://wiki.segger.com/RTT
origin of the vendored code: https://github.com/SEGGERMicro/RTT
The library is unmodified except for the CMakeLists.txt added.

I wrote this for myself, but if this is interesting I can definitely work on getting it merge ready!
(thanks to @stellar-aria for her existing RTT example)

TODO/Open questions:

• Can we exclude this code for applications that aren't using it? (ala fatfs)
• Makefile support
• Should I be using git submodules instead?
• Add an example to DaisyExamples (with vscode configuration)