metadata field fieldManageability set to SubscriberControlled

[nwcm]

If TAF is used in a managed package the metadata items cannot be disabled or bypassed in installed orgs. As all metadata fields are set as DeveloperControlled

This PR updates key control fields to be SubscriberControlled allowing installed orgs to bypass, or configure the permissions.

Changing DeveloperControlled to SubscriberControlled will not impact any existing package usage. However, do note if you have included it in a managed package any installed orgs will not get the new metadata config. Only fresh installs would have these fields as SubscriberControlled

It is possibly better to change all fields to SubscriberControlled to avoid issues since there is no official managed package

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

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