Make it easier to configure a wallet in a provider

[mattsse]

this currently requires

alloy/crates/provider/src/builder.rs

Lines 253 to 258 in 38060a6

	/// Add a wallet layer to the stack being built.
	///
	/// See [`WalletFiller`].
	pub fn wallet<W>(self, wallet: W) -> ProviderBuilder<L, JoinFill<F, WalletFiller<W>>, N> {
	self.filler(WalletFiller::new(wallet))
	}

 let wallet = EthereumWallet::from(signer);

which is usually initialized like this:

alloy/crates/provider/src/provider/trait.rs

Lines 1837 to 1839 in 38060a6

	let signer: PrivateKeySigner =
	"0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80".parse().unwrap();
	let wallet = EthereumWallet::from(signer);

it would be easier if this could be directly initialized which a Privatekeysigner.

to make this more ergonomic, we can introduce a helper trait that serves as a specialization for PrivateKeysigner so we can accept something like impl IntoWallet or similar.