Add ephemery testnet support

[syjn99]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

This PR adds a built-in support for the ephemery testnet, behind a new --ephemery flag.

Ephemery resets a fresh genesis every 28 days, so we need to fetch everytime at startup to collect artifacts:

• https://ephemery.dev/latest/config.yaml → parsed via params.UnmarshalConfig, activated with ConfigName: ephemery
• https://ephemery.dev/latest/boot_enr.txt → bootstrap ENRs (validated per line; an empty valid set is a startup error)
• https://ephemery.dev/latest/genesis.ssz → genesis state, via a new genesis.URLProvider, used only when neither --genesis-state nor --genesis-beacon-api-url/--checkpoint-sync-url is given

Which issue(s) does this PR fix?

N/A

Other notes for review

Documentation

After this PR is merged, I will update the "client implementation" section in ephemery-resources. So basically an entry for Prysm will be changed to "Complete" (Genesis Function Status).

Also we might dedicate a page for our documentation as well.

Live verification (with Geth)

# 1. Execution client on Ephemery (docker geth, initialized from the current iteration's EL genesis)
curl -sO https://ephemery.dev/latest/genesis.json
openssl rand -hex 32 > jwt.hex

docker run -d --name ephemery-geth -v $PWD:/cfg -p 8551:8551 -p 30303:30303 ethereum/client-go \
  init /cfg/genesis.json && docker restart ephemery-geth  # then run with:
# --authrpc.addr 0.0.0.0 --authrpc.jwtsecret /cfg/jwt.hex --bootnodes <BOOTNODE_ENODE_LIST from https://ephemery.dev/latest/nodevars_env.txt>

# 2. Beacon node from this branch
bazel run //cmd/beacon-chain -- --ephemery --datadir /tmp/eph-beacon \
  --execution-endpoint http://127.0.0.1:8551 --jwt-secret jwt.hex --accept-terms-of-use

With these commands, the beacon node SHOULD sync to the tip of the chain, from the genesis.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).

[syjn99]

Note: Ephemery's config stops at the last scheduled fork (currently Fulu). params.SetActive rejects a config whose later fork versions collide with mainnet leftovers, so the ephemery path fills omitted versions by numbering up from the genesis fork version (0x1000101b → 0x8000101b), logging a warning per filled fork.