Support BlockRef alongside ChainPoint in SDK helper methods

[yHSJ]

More flexible block identification was added in utxorpc/spec#154, allowing BlockRef to use any combination of slot, hash, height, and timestamp. This isn't made accessible to end users though, since the SDK helper methods (followTip, fetchBlock, fetchHistory, etc.) still only accept ChainPoint.

I would propose having these methods accept some union type of sync.BlockRef (or watch.BlockRef, I think they're the same) and ChainPoint, something like:

type BlockRefLike = ChainPoint | sync.BlockRef;

function blockRefLikeToBlockRef(p: BlockRefLike) {
  if (p instanceof sync.BlockRef) return p;

  return new sync.BlockRef({
    slot: BigInt(p.slot),
    hash: new Uint8Array(Buffer.from(p.hash, "hex")),
  });
}

async *followTip(intersect?: BlockRefLike[]): AsyncIterable<TipEvent> { ... }

This change would maintain backwards compatibility, if that's important here.