This commit should not have change in behavior. It changes the fulcio signer
to take in the KeyOpts struct instead of passing individual parameters through,
as well as moves common code dependent on the values to the same place.

The motivation behind this change is to make it easier to add new options without
needing to plumb through another param to the already long list.

To avoid circular dependencies (sign -> fulcio -> sign), KeyOpts was moved to the
options package.

Signed-off-by: Billy Lynch <billy@chainguard.dev>

Signed-off-by: Billy Lynch <billy@chainguard.dev>