The specification describes the signature token to be right after a newline
(\n) then the token "signature" and then a white-space followed by the encoded
signature.

This commit makes sure that when we parse the signature from the descriptor,
we are always looking for that extra white-space at the end of the token.

It will allow us also to support future fields that might start with
"signature".

Fixes #26069

Signed-off-by: David Goulet <dgoulet@torproject.org>