[Go] Bulk/Multi Read fields in ikv go client

[pushkarmoi]

Ikv go client only supports single get in IKVReader - ie a user can fetch one field from one document.
We should add the following batch/multi versions for each field type -

• Multiple primary keys, one field-name
• One primary key, multiple field-names

Tip: Minimize object allocation in the returned objects (ex. avoid maps)

Reference (in java client)
https://github.com/inlinedio/ikv-java-client/blob/32c10fc40e9397c7e6ced771a5768e6436244ef5/src/main/java/io/inlined/clients/DefaultInlineKVReader.java#L135