Add support for signature verification using public key in X509 DER encoding format.

[hansonchar]

Problem:

In addition to Exposing accessors for private/public key on elliptic curve keys, we need signature verification to support public key in X509 DER encoding format. This is similar to the issue addressed by #256 for key agreement, but for signature verification.

Solution:

The proposed solution is to build on top of #259, but add support for signature verification using public key not only in Octet String format, but also X509 DER encoding format.

• Does this change any public APIs? If yes, explain.

Adding functions:

• signature::UnparsedPublicKey.new_with_x509()

• Which algorithm(s) will this impact?
  RSA, ed25519 and EC

Requirements / Acceptance Criteria:

Signature verification works with public key provided in either Octet String format or X509 DER encoding format.

• Related Issues: Link any relevant issues

• Expose accessors for private/public key on elliptic curve keys #259

• Add support of Octet String and X509 bytes encoding (#255) #256

• Will the Usage Guide or other documentation need to be updated?

Yes

• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly
  interesting/important unit tests.

Unit tests added to cover RSA, EC and ed25519 for signature verification using public key in X509 DER encoding format. More unit tests will be added if this proposed pull request looks reasonable enough to be potentially accepted.

[justsmth]

This is an API change that we're not currently planning to make. We will reopen the issue if needed.