Saving snap-build assertion as '.build'.

[cprov]

It continue to support legacy filenames ('.snap-build') but issues a warning suggesting the renaming to the new extension.

This change was suggested as part of https://forum.snapcraft.io/t/isv-questions-about-signing-a-snap/2546/12.

[pedronis]

mmh, if we go with something that is not simply the type of the assertion as extension, it's a bit unclear to me that ".build" makes it very clear that it is some sort of signature

[cprov]

@pedronis yes, it's on hold until we find an agreement on the forum topic

[sergiusens]

Thanks @cprov, I am closing this until we have further agreement. I am only closing to make it easier to focus on the list of active PRs up for review.