feat: implement a walk function.

[peterhuene]

This PR implements a walk function that exposes the backends ability to walk a cloud URL for objects prefixed by the given URL.

It can be used to glob a cloud storage URL.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have added at least one relevant code reviewer to the PR.
• Your code builds clean without any errors or warnings.
• You have added tests (when appropriate).
• You have updated the README or other documentation to account for these
  changes (when appropriate).
• You have added an entry to the relevant CHANGELOG.md (see
  "keep a changelog" for more information).
• Your commit messages follow the conventional commit style.

[peterhuene]

CI failure is due to rust-lang/socket2#616, which hasn't yet made it to a release.

We can ignore the failure for now.

[claymcleod]

LGTM

[peterhuene]

Now we need getrandom to publish containing rust-random/getrandom#732. Going to just merge with the failing docs.