provide public API for unpacking a header without decrypting a message

[farleyb-amazon]

We have had requests from customers to be able to unpack the header without attempting to decrypt the message, in order to make a decision about whether or not decryption should be attempted or how that decryption should be configured.

Related:

• provide public API for unpacking a header without decrypting a message aws/aws-encryption-sdk-python#141
• provide public API for unpacking a header without decrypting a message aws/aws-encryption-sdk-java#96

[jalaziz]

Just ran into a need for this myself.

[seebees]

This does not yet exist for every runtime,
but you can use the ESDK-JS serialize package to do this

https://github.com/aws/aws-encryption-sdk-javascript/tree/master/modules/serialize
https://www.npmjs.com/package/@aws-crypto/serialize