[sim] Move pending base fee estimation to FeeEstimator from provider

[dancoombs]

Describe the feature
Chains are inconsistent in how they expose the pending base fee via their RPCs. See #821.

To handle this, we should implement base fee oracles (similar to our priority fee oracles) that use different RPC methods based on the chain. This oracle can be chosen via ChainSpec.

Known implementations needed:

• FeeHistory (default) - use eth_feeHistory and Alloy's FeeHistory::next_block_base_fee
• Constant - Provide a constant base fee (use for Flow)
• BaseFee - use eth_baseFee (use for Avalanche)