Add optional confirmation_depth field to cert verification

[epociask]

A confirmation depth should be added in the certificate verification as a counter-measure to an attack vector where the disperser gives a rollup sequencer a blob which is verified against a block which is subsequently reorged from the L1 chain. In the current DA interaction model, this would result in DoS on the rollup, rendering it incapable of progression state.

[teddyknox]

#51