Implementation: Encrypt input maxPlaintextSize is NOT passed to CMM is plaintext length is known

[lavaleri]

Change: https://github.com/awslabs/aws-encryption-sdk-specification/blob/master/changes/2020-06-17_encrypt-max-plaintext-length-input/change.md
The GetEncryptionMaterials call during Encrypt MUST use the actual plaintext length (if it can be determined) for max plaintext bound even if a maxPlaintextSize is supplied on input.

• Python Pass actual plaintext length to GetEncryptionMaterials during Encrypt over source_length aws/aws-encryption-sdk-python#282