to support GetConfigBlockFromOrdererByNumber

For an exist channel , when a new org need to be added to channel:
the channel's configBlock is updated first, then the org's peers join the channel

code may like as follows
```
//code update the configblock
...
...

//code to get the latest configblock
block, err = channel.GetConfigBlockFromOrderer(context.Background(), orderer0.Conn, orderer0.Signer, channelID, orderer0.KeyPair)
if err != nil {
logger.Error(err)
return err
}
//code to make peer join the channel
err := channel.JoinChannel(block, peerCli.Signer, peer.NewEndorserClient(peerCli.Conn))
if err != nil {
logger.Error(err)
return err
}
```
JoinChannel(...) will give an error: unsuccessful response received with status 500 (INTERNAL_SERVER_ERROR): cannot create ledger from genesis block: expected block number=0, received block number=1

So when a peer join the channl it will always need the number 0 ConfigBlock, is it correct?
but channel.GetConfigBlockFromOrderer(...) always return the latest ConfigBlock
so we need a func like channel.GetConfigBlockFromOrdererByNumber(0) or channel.GetFirstConfigBlockFromOrderer() to get the first ConfigBlock to support peer join