* Move GetRekorClient into util directory

Since other sigstore projects are using GetRekorClient, this moves it
into the pkg/util directory so that the number of dependencies this
brings with it can be minimized.

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>

* move to pkg/client

Signed-off-by: Bob Callaway <bob.callaway@gmail.com>