[Feature Request]: Make Avail ID a tuple of (block_height, extrinsic_index)

[0xRampey]

Implementation ideas

Currently, the da.ID implementation for AvailDA contains only the block height where a blob was posted. This is not an exact pointer to the blob and hence, the Get(ctx context.Context, ids []da.ID, namespace da.Namespace) method has to comb through all the DataTransactions in the block to find it. In fact, it looks like the Get method is currently returning all the blobs at the ID-specified block height.

Adding TransactionIndex to the ID implementation of an Avail blob should solve the above as it is unique per blob submission transaction and can be used to directly pinpoint and retrieve a blob with a specifc ID on Avail.