relayer: method to check that write tx confirmed

[tuxcanfly]

This PR adds a new method relayer.Check which takes a write transaction hash and returns the height at which the transaction was included in a block.

This is useful because we want to be able to pass the block height to Read and this fixes a hack which was in place in the tests.

[tuxcanfly]

In this PR we have added func Check(hash *chainhash.Hash) (int64, error) which returns the height at which the bitcoin transaction was mined. But we also need another index to track the data that we wrote, because there might more than one Write([]byte) that happened, probably from other callers.

This is why I had to re-order the tests because they'd fail with two rollkit-btc: gm messages rather than one, the reason being that the first Write is called from ExampleRelayer_Write(), and for testing the most recent write, we're calling Write again in ExampleRelayer_Read(). Even now if the test is ran twice in quick succession it can fail because the last block might contain a Write transaction from the previous test. 😢

The question is what should the second index be, should it just be the index of our transaction in the block? But that doesn't help much because Read() returns an array of data blobs, in order, as it reads all the transactions in the block. How would we know from the transaction index, which index of the blobs is ours? We'd need to scan the block for data to make that map.

For now though, Read() works because rollkit's da.RetreiveBlocks expects just that i.e all the blobs that were posted.

Ideally I think we'd have a map of map[txhash] -> blob and read back the exact data we posted, by getting the blob by our tx hash.

[tuxcanfly]

For reference, I think ordinals uses satid for this which is sort of like tx output i.e. transaction hash + output index.

[tuxcanfly]

Closing in favor of #33 where this can be reused.