Add featureFlag gateDecorator for sync-metadata

[Weiko]

Context

Fixes #2924
We want to start to work on email integrations and add new models. However, those models should not impact all workspaces during our implementations (which will take a few sprints) so we should be able to edit or create a standard object and sync-metadata command should skip those modifications.

Realistically, the only way I see it for now is to add a parameter to ObjectMetadata / FieldMetadata decorators, if the sync-metadata service sees this, it should skip the field or the object entirely, meaning it will consider it does not exist OR does not exist anymore and will do the changes accordingly, except for workspaces with the flag. This brings some limitations but should be enough for this use case

Implementation

As explained above, this PR introduces a new GateDecorator that we can add on our standard Objects/Fields. For now you can provide a featureFlag parameter and we will make sure to match it with feature flags stored in the DB. If the flag is not present, we skip the object or field creation.
This has some limitations but should be enough for our needs for now.
Note: For relations, make sure you also gate both directions

@ObjectMetadata({
  namePlural: 'messages',
  labelSingular: 'Message',
  labelPlural: 'Message',
  description: 'Messages',
  icon: 'IconMessage',
})
@Gate({
  featureFlag: 'IS_MESSAGING_ENABLED',
})
@IsSystem()
export class MessageObjectMetadata extends BaseObjectMetadata {}

&&

[...]
export class MessageThreadObjectMetadata extends BaseObjectMetadata {
[...]
  @FieldMetadata({
    type: FieldMetadataType.RELATION,
    label: 'Messages',
    description: 'Messages',
    icon: 'IconMessage',
  })
  @RelationMetadata({
    type: RelationMetadataType.ONE_TO_MANY,
    objectName: 'message',
  })
  @IsNullable()
  @Gate({
    featureFlag: 'IS_MESSAGING_ENABLED',
  })
  messages: MessageObjectMetadata[];

[magrinj]

Great, LGTM !