Add reactions option in activity endpoint

[homerzhm]

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Description of the pull request

We are using GetStream activity service. We want to fetch specific activity by Id.

When we using this SDK, it provide a function to get it.

However, we also want the activities data come back with reaction count, for example: Number of Like, Number of Comments.

I follow the same pattern for FlatFeed class.

I create ActivityReactionsOptions with the options that list in Endpoints. Activities Section of following link
https://getstream.io/docs_rest/

and also create ActivityEndpointParameters to make sure there is no mistype of parameter string.