Add eip6800 (verkle) feature to configs/feature.yaml

[marioevz]

Add eip6800 feature to the configuration file for the repo to be able to build and package releases with verkle fixtures easily.

In PR #573 we have an example of how this was accomplished for EOF, and we can similarly do this for verkle:

• Add CancunEIP6800 (or similar) to activate verkle in an isolated way, similar to our CancunEIP7692 fork for EOF. Although it could be possible that we need more than one fork for Verkle, e.g. to have different conversion strides.
• Add a eip6800 evm configuration to file configs/evm.yaml similar to eip7692 that we added to EOF.
• Add the correct config to tox.ini, like the one added for EOF.
• Optionally add and enable a tox verification matrix configuration for verkle in tox_verify.yaml.

[spencer-tb]

• Config, evm type and tox verification steps are added in this commit: 631a5e793a20086ed6eb35f479870db89a075222 of verkle/main

[spencer-tb]

Added the first release here: https://github.com/ethereum/execution-spec-tests/releases/tag/eip6800%40v0.0.1

I would ideally like to finalize the changes of verkle/main and merge them into main before we close this issue.