BFT Block Puller: Allow the orderer to supply a stream of header + signatures (attestations)

[tock-ibm]

The SeekInfo message is augmented with type enum SeekContentType and field SeekContentType content_type = 5;, as shown below:

message SeekInfo {
    enum SeekBehavior {
        BLOCK_UNTIL_READY = 0;
        FAIL_IF_NOT_READY = 1;
    }

    enum SeekErrorResponse {
        STRICT = 0;
        BEST_EFFORT = 1;
    }

    // <NEW> SeekContentType indicates what type of content to deliver in response to a request. If BLOCK is specified,
    // the orderer will stream blocks back to the peer. This is the default behavior. If HEADER_WITH_SIG is  specified, the
    // orderer will stream only a the header and the signature(s), and the payload field will be set to nil. This allows
    // the requester to ascertain that the respective signed block exists in the orderer (or cluster of orderers).
    enum SeekContentType {
        BLOCK = 0;
        HEADER_WITH_SIG =1;
    }
    
    SeekPosition start = 1;               // The position to start the deliver from
    SeekPosition stop = 2;                // The position to stop the deliver
    SeekBehavior behavior = 3;            // The behavior when a missing block is encountered
    SeekErrorResponse error_response = 4; // How to respond to errors reported to the deliver service
    SeekContentType content_type = 5;     // <NEW> Defines what type of content to deliver in response to a request
}

Note: the change the protobuf messages is already on the main branch.

When a stream of Header+Signatures is needed from the orderer, the peer (or any other client) will simply set the ContentType in the SeekInfo message to HEADER_WITH_SIG.
In this case, the orderer will simply set the block data to nil.

if seekInfo.ContentType == ab.SeekInfo_HEADER_WITH_SIG {
    block.Data = nil
}

Since a content type of BLOCK is the default value, no modification is needed to clients that currently request a stream of blocks.