Etherlink: FA Bridge feature flag in kernel

What

Adds a feature flag to enable the FA bridge.

Why

In order to add code to the FA bridge without modifying the current implementation.

How

If the path /evm/enable_fa_bridge has a value, the configuration says it's enabled. The following merge request should deal with this boolean to accept or not deposits.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR