Makes the extradata for the bedrock transition block
a public variable. This is useful for exporting the
value to be imported in other packages where assertions
can be made to ensure that the extradata is correct.

When fetching the `L2OutputOracle`'s configured starting
block number, the L2 block with that number can be
fetched and then we can assert that it has the magic
extradata.

```bash
$ cast call 0x02Ac1923480E7fd234e7F2Fd11596b9F20372374 'startingBlockNumber()(uint256)'
3855343
```

```bash
$ cast block 3855343 --json | jq -r .extraData
0x424544524f434b
```

Now check like so:

```bash
$ cast --to-ascii 0x424544524f434b
BEDROCK
```