signing key rotation support (for shipping binaries)

Right now fdroidserver does not take into account that APKs can be signed by multiple signers, due to signing key rotation (added with signing-scheme v3/v3.1).

This touches these data models:

• signer in index
• AllowedAPKSigningKeys in metadata

(there's also the idea to use signing key rotation for better reproducible build support: #1254)