transfer-hook: Move interface and example into subfolders

[joncinque]

Problem

The transfer hook interface and example program don't follow conventions by being in two different subdirs: transfer-hook-interface and transfer-hook-example.

Solution

Make them into token/transfer-hook/example and token/transfer-hook/interface.

Note that I want to make a super-simple example CLI for people to deploy, and it made more sense to put that in a transfer-hook/cli subdir, rather than also have transfer-hook-cli, which is why I'm doing this now.

[buffalojoec]

Clean diffs!! |-|/| 😂