[Bulk Subscribe] Define SDK Specification

[DeepanshuA]

Define SDK Spec for Bulk Subscribe.

Currently, we have few SDKs where Bulk Subscribe is implemented.
Evaluate, if a separate method bulkSubscribe/subscribeBulk is required or same subscribe method, with additional configurations will suffice.

[sadath-12]

For me it seems different method could make it more easier , since at dev side we would need to prepare the different responses also it might be clean to extend from there