Add EDV provider

[DRK3]

REST API and EDV models must be decoupled so as to allow the EDV models to be used in other storage providers.

• FormatProvider (done in feat: SDS EDV format provider #2210)
• Add concrete EDV document processor (encrypt/decrypt) implementation #2249
• Generalize EDV format provider #2273
• Add SDS REST provider #2208

[llorllale]

@DRK3

High-level Design

storage.Formatter: formats documents to and from storages in a way that preserves the user's privacy vs. external storage providers. This is what is currently known as edv.formatprovider.DocumentProcessor.

package storage

type Formatter interface {
   FormatPair(k string, v []byte) ([]byte, error)
   ParseValue([]byte) ([]byte, error)
}

edv.Formatter: implements storage.Formatter. Formats k and v into an EDV EncryptedDocument.

edv.Store: implements storage.Store. It will essentially be a thin wrapper around the EDV REST API.

storage.FormattedStore: implements storage.Store and is composed of an underlying store and a storage.Formatter. It formats all data before saving it to the underlying store

We will not have to add any new methods to storage.Store. We can delete storage.Store.Query().

Folder hierarchy:

• pkg/storage
  • formats
    • edv (EDV format impl)
  • edv (EDV Store impl)

[DRK3]

Support for the [https://github.com/trustbloc/edv](TrustBloc EDV Server) ReturnFullDocumentsOnQuery and Batch extensions have also been implemented in #2355/#2356 and #2365. These are optional capabilities that the EDV REST provider and formatted provider (respectively) can be initialized with.