Move to libnettle

[kbingham]

We only use a small subset of features given from gnutls, which are themselves provided by libnettle.

Move to use libnettle directly and remove the dependency on the whole of gnutls.

[imcoder-in]

@kbingham can i start looking into this issue ?

[kbingham]

Oh wow - this is old - but sure! It is likely still appropriate. It probably needs checking in with Laurent Pinchart to see if he still thinks it's appropriate. Best way to talk directly to the team is on the IRC/Matrix channel https://matrix.to/#/#oftc#libcamera:matrix.org