effectively, same thing as `const __FlashStringHelper*`
but, we know the actual size at build time vs. calling `strlen_P`
can't specialize the way things are read until v6; current implemenation
simply attaches arbitrary char pointer to the internal buffer, which is
then read byte-by-byte when writing output json stream
revert to static json buffer class in `/discovery`
will move more things to it, but probably should move to v6 first