Support Rekor v2 logs

[haydentherapper]

Description

The read API has significantly changed between Rekor v1 and Rekor v2. Entries in the log are stored as "tiles" and served per https://github.com/C2SP/C2SP/blob/main/tlog-tiles.md#log-entries. Entries are served as "entry bundles", a set of entries for each tile in the lowest level of the tree. When reading entries from the log given a log index, it is the client's responsibility to compute which entry bundle the client needs to request and the index of the entry in the bundle.

What this means is:

• Retrieval by index is now the responsibility of the client, as /api/v1/log/entries?logIndex= has been removed and replaced with the above API
• Retrieval by leaf hash (entry UUID) is no longer supported. Entries are identified exclusively by log index.
• The search API /api/v1/index/retrieve has been removed. We are investigating spinning up a separate service for this. If you want to retrieve an entry by email, artifact hash or commit SHA, you would need to build a search index.