Use stdToml where possible, custom functions elsewhere

[alcueca]

Description

There is a suggestion from @mds1 about using stdToml instead of the more raw parseToml commands.

I took a stab at it. In the current repo, we would use stdToml exactly twice, because most of the time we are parsing structs that stdToml can't manage.

We could extract the struct-reading abi.decode command into its own function. Some of them would be reusable such as AddressesRegistry.readChainsFromToml in this PR. Most of them would be template-specific.

@mds1, worth it to fix the other templates in the same way as TransferOwnerTemplate here, or should we let it be?

[mds1]

Sorry, I missed this, but am supportive of this PR once conflicts are resolved

[mds1]

We can even hardcode the key here since it will always be the same

Suggested change

	function readChainsFromToml(string memory toml, string memory key) public view returns (ChainInfo[] memory) {
	return abi.decode(vm.parseToml(vm.readFile(toml), key), (ChainInfo[]));
	}
	function readChainsFromToml(string memory toml) public view returns (ChainInfo[] memory) {
	return abi.decode(vm.parseToml(vm.readFile(toml), ".l2Chains"), (ChainInfo[]));
	}