feat: Improve the `GasOracle` usage

[shekohex]

Summary of changes

• Add a new option to override the default API Endpoint for Etherscan API configuration (see below for an example)
• Adds a new feature that will use different gas oracle at once and take the median value of all of the succeeded gas oracles in a weighted way. For example, we will query in parallel all configured Gas Oracles and take the median value of all of the responses. For now, we support the following gas oracles:
  1. Native Provider Gas Oracle (a la eth_gasPrice RPC) with weight of 0.1 and this the default gas oracle.
  2. Etherscan Gas Oracle with weight of 0.9 and it is optional by default.
• Updates webb-rs to reflect the new updates to ethers-rs to the latest commit to fix https://github.com/gakonst/ethers-rs/issues/2324

Example of the new Configuration options

[evm-etherscan.polygon_mumbai]
chain-id = 80001
api-key = "$POLYGONSCAN_API_KEY"
# Notice: We override the default API Endpoint
# from https://api-testnet.polygonscan.com/api
# to https://api.polygonscan.com/api
# To use the mainnet API instead of the testnet one
# Since the etherscan testnet's API does not support gas oracles.
api-url = "https://api.polygonscan.com/api"

Reference issue to close (if applicable)

• Depends on https://github.com/webb-tools/webb-rs/pull/131

---

Code Checklist

• [x] Tested
• [x] Documented

[shekohex]

Works now after all these updates:

{"estimatedFee":"0x3ddaea4a1bf2d5803c80","gasPrice":"0x8550417a9998a0000","refundExchangeRate":"0x6705e72f098f5c0000","maxRefund":"0x958da5c9b230a","timestamp":"2023-04-05T15:36:36.485408282Z"}