Add getter functions on PreparedRecord and PreparedDelete

[nicklaswj]

This PR add the following functions:

PreparedDelete::protected_indexes(&self) -> &[(Cow<'static, str>, IndexType)]

PreparedRecord::unsealed_indexes(&self) -> &[UnsealedIndex]
PreparedRecord::protected_indexes(&self) -> &[(Cow<'static, str>, IndexType)]

This basically allows me to fully mock cipherstash_dynamodb::EncryptedTable which we need for CI purposes.

[coderdan]

@nicklaswj thank you. FYI moving forward we will require all commits to be signed before merging. I'll merge this PR as is for now.