Implement dummy header for testing

[tzdybal]

To ensure that tests are not depending on ExtendedHeader, some DummyHeader implementation has to be provided. This code will be resolved by celestiaorg/celestia-node#1304, because tests needs to be done before we can merge the changes in node.

Definition of done: all the tests in pkg/headerare passing.

[tzdybal]

This issue is resolved by https://github.com/tzdybal/celestia-node/tree/297b59eb8428cb031b7e307d2fea1ceb04bbf2fa

Rebasing and final review of changes is blocked (see #593)