Add eth_feeHistory based fee estimation

[alisinabh]

• New Ethers.fee_history/4 (+ bang variant) wrapping eth_feeHistory with all quantities decoded to integers; also usable in Ethers.batch/2 via {:fee_history, [block_count, newest_block, reward_percentiles]}
• New Ethers.estimate_fees/1 (+ bang variant): estimates EIP-1559 fees from the median of recent blocks' priority fees sampled at a percentile selected by :speed (:slow/:standard/:fast or a raw percentile), with max_fee_per_gas = 2 * next_base_fee + max_priority_fee_per_gas
• Transaction auto-fill now prices missing max_fee_per_gas / max_priority_fee_per_gas through estimate_fees instead of a margin over eth_gasPrice, falling back to the legacy estimation on RPC clients without eth_feeHistory support (the new adapter callback is optional)

[Copilot]

Pull request overview

This PR introduces eth_feeHistory-based fee estimation to improve EIP-1559 transaction pricing, exposing a decoded Ethers.fee_history/4 API, a higher-level Ethers.estimate_fees/1 helper, and updating transaction auto-fill to prefer these estimates with a legacy fallback when unsupported.

Changes:

• Add Ethers.fee_history/4 (+ fee_history!/4) and support batching via {:fee_history, [...]}.
• Add Ethers.estimate_fees/1 (+ estimate_fees!/1) to derive EIP-1559 fee caps from recent blocks’ priority fee samples and next base fee.
• Update EIP-1559 transaction auto-fill to use estimate_fees/1 when supported, falling back to legacy gas price heuristics otherwise, and add test coverage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/ethers/fee_estimation_test.exs	Adds unit/integration tests for fee history decoding, fee estimation math, batching, and transaction auto-fill fallback behavior.
lib/ethers/transaction.ex	Switches EIP-1559 fee auto-fill to prefer estimate_fees/1 when available and retain legacy fill actions as fallback.
lib/ethers/rpc_client/adapter.ex	Adds optional eth_fee_history/4 callback to keep custom adapters backward compatible.
lib/ethers.ex	Implements fee_history/4, estimate_fees/1, decoding/post-processing, and percentile/median helpers.
CHANGELOG.md	Documents the new APIs and changed transaction auto-fill fee behavior.

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