Generalize SignedCheckpoint to take arbitrary Notes (#347)

* generalize signed checkpoint

Signed-off-by: Asra Ali <asraa@google.com>

* store note as text representation

Signed-off-by: Asra Ali <asraa@google.com>

* cleanup diff

Signed-off-by: Asra Ali <asraa@google.com>

* simplify

Signed-off-by: Asra Ali <asraa@google.com>

* use signer/verifier

Signed-off-by: Asra Ali <asraa@google.com>

* address dan comments

Signed-off-by: Asra Ali <asraa@google.com>