Add TryGetReminder to actor SDK

[WhitWaldo]

Describe the feature

It's useful to know if a reminder already exists or not. Matching the syntax of new state methods, adding the TryGet* method here maintains consistency and provides definite feedback as to whether a given reminder already exists or not (without deciphering what null value means, especially as this package doesn't support nullability annotations).

This returns a ConditionalValue which contains a HasValue boolean property and a Value TValue property. If there's a reminder available, HasValue will be set to true and the Value property will contain the reminder info. If not, HasValue will be set to false and the Value property will be populated with a null value.

Release Note

RELEASE NOTE: ADD Added TryGetReminder method to return whether the reminder exists or not and its value, if it does.