Refactor Step 1 - move "padded" cipher implementation to sub-module

[justsmth]

Description of changes:

• Without changing the public API, this change moves much of the implementation for padded block ciphers to its own sub-module.
• Prepare for the use of EVP_CIPHER_CTX for streaming operations:
  • Refactor of UnboundCipherKey to delay the creation of the SymmetricCipherKey (or EVP_CIPHER_CTX until it's "bound" to an operation-specific key.
• This is the start of the work toward supporting for a "streaming" cipher modality (re: #396).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[justsmth]

API-diff now confirms that no API has changed: https://github.com/aws/aws-lc-rs/actions/runs/8847917376/job/24296754264?pr=408

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