Use const pointers in Stream::find() and Stream::findUntil()

[bblanchon]

Changed the signatures of Stream::find() and Stream::findUntil() to take const pointers, as they don't change the target.

Old signature	New signature
find(char *target)	find(const char *target)
findUntil(char *target, char *terminator)	findUntil(const char *target, const char *terminator)

All the overloads have been changed accordingly.

This should not cause any code to break.

[bblanchon]

👋 Can someone have a look?

[facchinm]

Hi @bblanchon ,
we are closing all the API related PRs in this repo.
If you are still interesting in getting this merged could you reopen in https://github.com/arduino/ArduinoCore-api ? Here you can find some hints about reapplying the patch to that repo easily.
Thanks!