[WIP] many: support multi-volume gadget asset updates + ICE

[anonymouse64]

Opening to share the state of the code, as well as to share the branch that was used to build beta/wendover branch. This includes all of https://github.com/snapcore/snapd/pull/10906 as well as https://github.com/snapcore/snapd/pull/11575 as well as an integration commits making ICE work with multi-volume asset updates that needs a bit of work and more tests (and to fix existing tests too): 017ad1ad4593d5f71ce3ffbb0023c3f04ad9e816