EIP-7516 Blobgasfee

Documentation for tests/cancun/eip7516_blobgasfee@7b8124a7.

Generate fixtures for these test cases for Bpo2 with:

fill -v tests/cancun/eip7516_blobgasfee --fork Bpo2

Tests for EIP-7516: BLOBBASEFEE opcode.