Add props support to player relationship subscriptions

[tudddorrr]

Subscription props

• SubscribeTo now accepts optional props, so subscriptions can carry metadata at creation time.
• Added UpdateSubscriptionProps to update the props on an existing subscription.
• Subscriptions now extend EntityWithProps, exposing their props like other entities.

Results and validation

• Both calls return a SubscriptionUpsertResult reporting success, the subscription, and any rejected props.
• Rejected props are parsed from bad request responses; other errors are still thrown.
• Props are sanitised before being sent to the API.

Friend requests sample

• The Friends demo now checks result.Success when sending a friend request.