Configurable GSA fix_stas_prn size

[elpiel]

Testing with the Quectel LC76G GNSS receiver which supports the GNSS providers:GPS, GLONASS, Galileo, BDS, QZSS was causing an error for GSA.

[elpiel]

So far, from the logs, I cannot find any sentences with more than 10 prns,
There are 2 possibilities:

1. There are empty ones that don't show in the parsed sentence, i.e. ,,,,
2. The many0 parse that we've copied from nom and substitute the std::vec::Vec with heapless::Vec is causing a problem when parsing the sentences