Use Polygon gas oracle

[tkporter]

Description

After investigating #2959, I found the following

this is a known problem with Polygon, explanation here ethers-io/ethers.js#2828 (comment)

Fun fact Asa looked into this once #771

Here's a discussion it in Foundry, which I found hoping that ethers-rs folks had ran into this before
foundry-rs/foundry#1703

Foundry fixed this by using the Polygon gas station oracle which seems to be recommended path https://github.com/foundry-rs/foundry/pull/3368/files#diff-c89a4bbf7a90da118dcf00c5fe70eba78f8e5d95662bb5f039a353113e95042bR205

There's actually a polygon ethers middleware for this https://docs.rs/ethers/latest/ethers/middleware/gas_oracle/polygon/struct.Polygon.html

So I (originally) borrowed this code from Foundry https://github.com/foundry-rs/foundry/blob/master/crates/common/src/provider.rs#L254-L290

Changed to use Middlewares

This also means we can remove our existing janky Polygon logic

Drive-by changes

Related issues

Backward compatibility

Testing

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[yorhodes]

nice work
would be good to just import as a middleware and add to the stack

[tkporter]

Changed to use middlewares - note this was a bit more painful / ugly than you'd probably expect, which is a result of the Middleware trait not being object safe. So conditional usage of a middleware gets really messy (see gakonst/ethers-rs#592)

The GasOracleMiddleware will apply the provided GasOracle regardless of what chain it is, so we'd need to either:
a) conditionally use the GasOracleMiddleware, or
b) because GasOracle is object safe, just always use a GasOracleMiddleware but use a different GasOracle depending on the chain

With (a), we end up with a bunch of really ugly conditional paths because we always need a concrete type for the provider/middleware object

And with (b), we need to have a GasOracle that will signal to the GasOracleMiddleware to not use a gas oracle at all - they have ProviderOracle, which lets you use the provider's oracle instead. This is fine, but the downside is to make this work we have to make the provider Arc<M> bc ProviderOracle requires ownership of the provider