go/common/node: Add custom text (un)marshaler for RolesMask type

[tjanez]

This will result in easy to understand "roles" fields in various CLI commands that output JSON.

For example, instead of oasis-node control status showing:

{
  ... trimmed ...
 
  "registration": {
    "last_registration": "2021-09-02T13:25:53+02:00",
    "descriptor": {

      ... trimmed ...

      "roles": 24
    },

  ... trimmed ...
}

it will show:

{
  ... trimmed ...
 
  "registration": {
    "last_registration": "2021-09-02T13:25:53+02:00",
    "descriptor": {

      ... trimmed ...

      "roles": "validator,consensus-rpc"
    },

  ... trimmed ...
}

[kostko]

Should we reject duplicate roles when parsing?

[tjanez]

Should we reject duplicate roles when parsing?

Agreed. Done.