Support streaming API for CBC

[fgaud]

Problem:

I would like to be able to use CBC in a streaming fashion. Unfortunately, currently the CBC encryptor always puts padding after an encryption operation and expects padding during a decryption operation.

OpenSSL solves that problem with finalize methods (for ex. https://docs.rs/openssl/latest/openssl/envelope/struct.Open.html#method.finalize)

Solution:

There are two possible options

• Add CBC support to EncryptingKey and DecryptingKey types, and have the caveat that the inputs must be divisible by the block size, and leave padding decisions to the caller.

• Introduce a StreamingPaddedBlockEncryptingKey and StreamingPaddedBlockDecryptingKey that implements the streaming and padding concepts.

The later would be nicer but more work

• Does this change any public APIs? No it add more APIs
• Which algorithm(s) will this impact? CBC

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete?

Ideally we can encrypt/decrypt in a streaming fashion.

• RFC links: N/A
• Related Issues: None
• Will the Usage Guide or other documentation need to be updated? Maybe depending on the solution
• Testing: How will this change be tested? Call out new integration tests, functional tests, or particularly
  interesting/important unit tests. Probably through unit tests

Out of scope:

Is there anything the solution will intentionally NOT address?

[justsmth]

FYI -- this API is now available in v1.8.0.