Fix `cursor` reset issue and cancelled `effects` spawning

[parfeon]

feat(subscription): add more types to create SubscriptionCursor from

Make it possible to create SubscriptionCursor from the string slice.

feat(subscription): add methods to manipulate subscriptions in set

Add add_subscriptions(..) and sub_subscriptions(..) to SubscriptionSet to make it possible in addition to sets manipulation use list of subscriptions.

fix(subscribe): fix cursor reset issue

Fix issue because of which cursor is not reset on Subscription and SubscriptionSet on unsubscribe.

fix(subscription): fix issue with cancelled effects

Fix issue because of which cancelled effects still asynchronously spawned for processing.

docs(inline): change client to pubnub in inline docs

refactor(subscribe): add subscription token validation

Added a method to validate the provided subscription token to conform to PubNub time token requirements with precision.

refactor(examples): refactor subscribe example

Separate subscribe example into two to show separately subscribe feature and presence state maintenance with subscribe.

[parfeon]

@pubnub-release-bot release

[pubnub-release-bot]

🚀 Release successfully completed 🚀