Confirm sidechain blocks when importing instead of when proposing

[murerfel]

Part of #855

As a preparation step for integritee-network/pallets#76, we need to change the workflow of sending sidechain block confirmations to the sidechain pallet.

In its current form, we compose an extrinsic with the confirmation (here, called OpaqueCall) and add it to the list of extrinsics to be sent here.

Now we need to change the workflow: This extrinsic should be composed and sent when IMPORTING the sidechain block.
We ignore the fact that the extrinsic is still called proposed_sidechain_block, which will not be accurate anymore. Re-naming this pallet call will be done in integritee-network/pallets#76.

Sending the block import confirmation can be done on this line.
This will most likely require some refactoring, since the importer then needs a way to compose and send the extrinsic. This may warrant introduction of a new component that composes and sends that extrinsic (and depends on the metadata provider)