Missing data key caching examples

In the docs, we have data key caching examples for Java and Python, but not C. We need to create and test these for C, then add them to the docs.

https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/implement-caching.html#caching-example-encrypt-string

https://docs.aws.amazon.com/encryption-sdk/latest/developer-guide/sample-cache-example.html

Review the linked data key caching documentation and compare its Java and Python examples to identify the expected C example structure. Create and test the C caching examples, then add the tested examples to the relevant documentation; done means the C examples work and appear alongside the existing language examples.

Written by the indexing model from the issue text.