Implement the Collection Exists Function

[shahzadlone]

Implement the cli and http Collection Exists Function that already has a public method on client.Collection (in client/collection.go).

It should respect ACP logic and only show true for documents that exist and are accessible.

Here is the public function we have so far (after acp is merged):

// Exists checks if a given document exists with supplied DocID.
//
// Will return true if a matching document exists, otherwise will return false.
Exists(ctx context.Context, identity immutable.Option[string], docID DocID) (bool, error)