go conversations: add support for conversations.inviteShared

[AChelikani]

Taken from slack-go#1195.

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[ChenJesse]

hmm the API docs are confusing, it says

Optional email to receive this invite. Either emails or user_ids must be provided. Only one email or one user ID may be invited at a time.

but the type of the arguments is array

https://api.slack.com/methods/conversations.inviteShared

[AChelikani]

Oh true I ran into this lol in our code it only goes one at a time. Gonna leave as is but the call side already loops and calls one by one right now.

[ChenJesse]

looks fine but idk if we can actually pass in multiple emails or user_ids