dynamic schema attribute support

[tomtau]

We'd like to be able to add a dynamic "catch-all-other-attributes" field in a data struct, e.g.:

#[derive(
    Debug,
    Clone,
    PartialEq,
    Eq,
    Searchable,
    Encryptable,
    Decryptable,
    Hash,
    PartialOrd,
)]
#[cipherstash(sort_key_prefix = None)]
pub struct Data {
      #[cipherstash(plaintext)]
      pub(crate) plain: String,
      pub(crate) protected: String,
      pub(crate) other_data: BTreeMap<String, String>,
}

where other_data contains all other attributes (the key is the attribute name, the value is the corresponding protected attribute value).
Encryptable and Decryptable traits cannot be currently derived for BTreeMap. If we try to implement them manually we'll hit two issues:

1. fn protected_attributes is static
2. a way to list these other attributes in fn from_unsealed : I guess one could use unsealed.protected() and exclude the statically known attributes?

[auxesis]

Thanks for reporting this @tomtau.

We think this is feasible, and will get back to you shortly with a rough timeline.

[tomtau]

@auxesis maybe one alternative for this, possibly safer, could be if Cipherstash supported nested DDB map, so it could just be one static attribute other_data (of type map), and the values inside that map would be protected.