api/index/retrieve: allow searching on indicies with sha1 hashes (#499)

Adds the ability to search for indicies with sha1 hashes. Currently
rekor custom types can store indices with formats other than
sha256:<hash>.  Particularly the in-toto type can do this.

One particular use case of interest is indexing log entries by git
commit hash, which largely still use sha1.

Signed-off-by: Mikhail Swift <mswift@mswift.dev>