Initial setup: Add structure, README, and Fusaka index for metric standardization

[will-corcoran]

This PR sets up the EL metrics repo to mirror beacon-metrics:

• Added README with purpose, usage, and hard fork index.
• Created fusaka/fusaka-index.md for Fusaka EIPs (Osaka EL focus).
• Includes proposed EL metrics for PeerDAS/GetBlobsV2.
• Aims for unified monitoring across layers.

[SunnysidedJ]

Adding corresponding PR from CL side: ethereum/beacon-metrics#19

Could we discuss structuring by functional scope instead of Fork/EIPs?
Re-pasting part of Sunnyside Lab's previous comment here.
[structuring by Fork/EIP] doesn't seem to be that user-friendly. 1) Some metrics may be common or generic across different EIPs. 2) Following different forks for all the change history would be another pain point. Consider Ethereum in 5 years: the number of forks to follow will double (the specs repo). Only looking at metrics of functionality they debug seems to be more intuitive approach for users imo. Also, most client teams seem to structure it by functionality as well.