Relayer accounts management

### Problem description

The current relayer account feature assumes the user can setup a relayer account for its own use.

While somehow useful from an accounting perspective, this approach does not cover all those cases where a platform owner, a platform provider, a centralized payer, etc, wants to pay for other users's transactions.

### Requirements

- The Guardian should allow the configuration of relayer accounts from configuration file, using a _firewall rules fashion_, something like this:
- For each relay account define permissions for each users, plus an "everyone" wildcard. The options will be:
- "_Must use_": relayer account visibile in the UI, users cannot deselect or add/use another relayer
- "_Can use_": relayer account visible in the UI, users can select and use it
- Rules order matter, each line overwrite the previous rules, for example:
- Everyone must use 0.0.123
- Bob can use 0.0.234
- Alice can use 0.0.345
- Carl must use 0.0.456
- Summary: Carl must use 0.0.456, Alice can use 0.0.345 but if she choose not to do it she is going to use 0.0.123, same for Bob he has the option to use 0.0.234 or the mandatory 0.0.123
- As a user, I can decide to use a relayer account regardless of what the registry or the policy developer wants. That configuration should be a user option and/or system admin choice.