Add signedEntryTimestamp to LogEntry for offline verification (#291)

* Add signature to LogEntry for offline verification

Also add an integration test for this.

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Address code review comments:
- Canonicalize payload before signing it
- Change name of signature to signedEntryTimestamp
- move signedEntryTimestamp and inclusionProof into separate Verification field in LogEntry

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Create helper func for extracting log entry

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Add info around verifying signedEntryTimestamp as comments in openapi.yaml

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>

* Generalize verification instructions

Signed-off-by: Priya Wadhwa <priyawadhwa@google.com>