[Feature Request] Support accessing raw last heartbeat details

[cretz]

Describe the solution you'd like

Users may need access to the raw form of the last heartbeat details (e.g. if they want to provide hints via #271). We should expose this, probably as a collection of Temporalio::Converters::RawValue on Temporalio::Activity::Info. Also need to make heartbeat_details lazy somehow because those using raw may not want to have eager deserialization.