feat: move export to dedicated module

[phisco]

Description of your changes

This PR:

• Moves the migration code to a dedicated Go module
• Reimplements its archive related code to avoid importing github.com/mholt/archiver/v4
• Makes its spinner pluggable, defaulting to a no-op

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] ~Added backport release-x.y labels to auto-backport this PR, as appropriate.~

How has this code been tested

The output of an export using the code from main:

The output of an export using the code from this PR:

The diff between the two generated archives using pkgdiff foo.tar.gz main.tar.gz, returned only timestamps differences.

Import:

$ kubectl create secret generic super-secret-secret --from-literal=password=supersecret
$ go run github.com/upbound/up/cmd/up alpha migration export --output foo.tar.gz
$ go run github.com/upbound/up/cmd/up alpha migration import -i foo.tar.gz
$ k get secret super-secret-password -o yaml