add point decompression check

[mattsb42-aws]

• when decompressing the ECDSA public key, DefaultCMM MUST perform the check described in Check that Y is a root of Y^2 in point decompression routine aws/aws-encryption-sdk-python#113
  • NOTE: None of our implementations currently do this

[mattsb42-aws]

this is in line for spec v1.1 once we create that milestone