feat: add isthmus spec

[leruaa]

Just adding HOLOCENE and ISTHMUS to OpSpecId.

It will be needed for ethereum-optimism/specs#373, and more precisely I would need it to be able to handle Isthmus in op-reth.

Closes #1874

[codspeed-hq]

CodSpeed Performance Report

Merging #1938 will not alter performance

_{Comparing leruaa:add-isthmus-spec (32dfc3a) with main (03ea24b)}

Summary

✅ 7 untouched benchmarks

[rakita]

lgtm! Thank you

[rakita]

Reth still uses release/v51 branch and is unsure if integration with main will happen before istmus. ISTHMUS needs to be added there

[leruaa]

@rakita Isn't reth on release/v50 branch? Trying to patch with release/v51 I hit changes related to Pectra

[rakita]

@rakita Isn't reth on release/v50 branch? Trying to patch with release/v51 I hit changes related to Pectra

It uses release/v50, but I will bump release/v51 soon, so new changes should be there.

It is next release after v50