Expand the amount of roles

[GregTheGreek]

(Solidity repo didn't have discussions setup)

I was thinking about access control in regards to the admin and was wondering if we could offer more granular access control on specific actions. Currently we have:

• RELAYER_ROLE - Given to accounts that are allowed to relay votes
• DEFAULT_ADMIN_ROLE - Given to an account that has access to special functions

Given the variability of gas fees on Ethereum, there is reason that someone may want to adjust the bridge fee (fee taken on deposits) on a more regular interval to accurately cover the costs of relaying transactions. This would mean that the same account that would be calling adminChangeFee() would also be have the ability to call adminWithdraw(). Assuming someone would want to automate the process of adjusting the bridge fee, this means a privatekey with immense amount of power would have to be exposed "somewhere". Typically after the initial setup, the admin is exchanged for a multi-sig or similar, which makes changing the fee a burden.

I'm curious to hear peoples thoughts on adding an additional role, FEE_MANAGER_ROLE, to which a modifier would be added to the adminChangeFee() allowing an account with that role to only manage the fee and nothing else.

This could also be applied to accounts adding resources, such that the multi-sig holders don't need to constantly vote to add a new token, nft, or similar.