Create validation logic for known metadata fields in guardian Metadata config

[alexlwn123]

There are some footguns with configuring federation metadata. For all core protocol metadata config options defined here, we should have validation logic to ensure the guardians inputs are properly formed.

e.g.

{
  ...
  "vetted_gateways": "31637A7863323431323361736466..."
  // Invalid - should be an array of hex encoded ids
}


{
  ...
  "vetted_gateways": [ "31637A7863323431323361736466..." ]
  // Valid!
}

In addition, there are some other metadata fields required for 3rd party clients (namely Fedi) that we should validate as well. For example, if a federation has the StabilityPool module enabled, they must update some 2 metadata fields for clients to show the feature.

e.g.

{
  ...
  "fedi:stability_pool_disabled": "false",
  // Invalid - missing "max_stable_balance_msats"
}


{
  ...
  "fedi:stability_pool_disabled": "false",
  "fedi:max_stable_balance_msats": "1000000000"
  // Valid!
}

[elsirion]

These are all Fedi-specific meta fields. Can we please keep any specific logic in a separate file at least? My long-term goal is to kinda allow every integrator to bring their own extra meta field validation rules/list of fields to select from in their own namespace (e.g. here fedi:).

[alexlwn123]

My long-term goal is to kinda allow every integrator to bring their own extra meta field validation rules/list of fields to select from in their own namespace (e.g. here fedi:).

Yeah, would be cool to have a "load metadata template" feature.

Didn't realize these were scoped to fedi. Will update the issue accordingly