install: Make stateroot configurable

[omertuc]

(draft because untested yet, also want to add integration tests)

This commit makes it so that the bootc install stateroot will be configurable (it defaults to default). For now this is a hidden CLI option until we decide whether we want to commit to this API.

In the future we also want to make the stateroot of bootc switch be configurable (https://github.com/containers/bootc/pull/617) so that users can install an image to a new stateroot while they already have an existing stateroot

Also removed the constant STATEROOT_DEFAULT, we're now simply taking it from the ostree_ext crate

[cgwalters]

For tests, try adding a new case to https://github.com/containers/bootc/blob/b5f47ff8fe5121f7eade46aa53048edd75fd6116/tests-integration/src/install.rs#L68

[omertuc]

Looks sane to me, could just use a test in tests-integration/src/install.rs or so.

Coming back to this, is this still the right place? Or should it be part of the e2e tmt stuff?

[cgwalters]

Coming back to this, is this still the right place? Or should it be part of the e2e tmt stuff?

the e2e-tmt flow is generally oriented around bootc as a client flow, which is currently pretty distinct from the install path. I think it's still the right place to test install code in that file.