Add Avalanche method spec with eth_baseFee

[pxlvre]

Summary

Fixes #273.

Avalanche's chains.yaml entry had no method-spec override, so it fell back to the bare eth spec, which never listed eth_baseFee — a real, documented Avalanche C-Chain JSON-RPC extension for the next block's base fee (docs). Every configured upstream serves the method correctly; only the spec-level allowlist was missing it, so nodecore refused to route it to any upstream and returned -32601 method does not exist with response-provider: NoUpstream.

This mirrors the existing fantom pattern (which adds ftm_effectiveBaseFee the same way) rather than introducing anything new:

• pkg/methods/specs/avalanche.json (new) — "spec-imports": ["eth"], adds eth_baseFee as a non-cacheable method (it reflects the next block, so caching it would be wrong for the same reason eth_gasPrice and ftm_effectiveBaseFee are also cacheable: false)
• chains.yaml — sets method-spec: "avalanche" on the avalanche entry
• docs/method-specs.md — lists avalanche alongside the other plain, eth-importing specs
• pkg/methods/methods_spec_test.go — new TestAvalancheSpecLoads

Test plan

• go test ./... passes (full repo suite, including the new test)
• Sanity-checked end-to-end against a local drpcorg/nodecore checkout (go.mod replace pointing at this branch): specs.GetSpecMethodsByConnectors("avalanche", ...) now includes eth_baseFee alongside the inherited eth methods (e.g. eth_blockNumber), and does not leak ftm_effectiveBaseFee or other unrelated chain extensions