Rewrite all samples using structs for activities

[alexshtin]

Activities can be implemented in a few different ways. The most robust and useful way is to implement them as structs (See temporalio/sdk-go#4). Samples still use legacy way and implement activities as single funcs.

All samples should be rewritten with structs.

[mfateev]

I have it already done. Waiting for a server release to submit: https://github.com/mfateev/temporal-go-samples/tree/struct_activities