Add support for signing and retrieving public key from KMS (#267)

* Add signing package for signing within rekor

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

* Remove public key from trillian and add in TODO for getting public key from Signer

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

* Create signer flag and store signer in api struct

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

* Remove public key from tlog in API, replace with a new pubkey tag

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

* Make sure we can get the public key locally

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

* Fix build error

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

* Reuse cosign implementation of signing interface

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

* fix lint

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

* Add in-memory signer, store unmarshaled public key in api

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

* Temporarily skip the log_info test, since we are now getting the public key from rekor and not trillian

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

* Replace cosign import with sigstore

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

* Add unit test for memory signer

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

* Remove unnecessary code

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

* skip test

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

* Sign the signature for the signed log root ourselves

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

* Include memory as a signer option for signer flag, make memory default

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