Add option for specifying trust key for signing schema1 manifests.
Since schema1 signature key identifiers are not verified anywhere and deprecated, storing signatures is no longer a requirement.
Furthermore in schema2 there is no signature, requiring the registry to already add signatures to generated schema1 manifests.
Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Create signedManifestHandler and schema2ManifestHandler. Use these to
unmarshal and put the respective types of manifests from manifestStore.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
