[v2] Separate proposed input types from the type-specific schemas

[asraa]

Description

Clients should allow sending proposed entries as non strongly types entries, something like the following:

ProposedEntry:

type: intoto
pk: pk
etc...

that includes the proposed type it intends to be. Rekor server-side should create the type specific entry.

This would provide clarity in the required fields of the server-side computed entries, and allow clients to not have to deal with type specific canonicalization.

I think search, create, retrieve should all use this type of input data.