Add basic support for activities

[freya022]

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Changes

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Closes Issue: NaN

Description

Adds basic support for activities:

• Creating/reading entry point commands (PrimaryEntryPointCommandData)
• Receiving/responding to entry point interactions (PrimaryEntryPointInteractionEvent)
• Reading the activity instance (InteractionCallbackResponse#getActivityInstance() from InteractionHook#getCallbackResponse()) created by ILaunchActivityCallback#replyLaunchActivity()
• Retrieving an activity instance by ID (JDA#retrieveActivityInstanceById(String))

[DV8FromTheWorld]

Nice work. Dropped a number of comments, but many are duplicates so when a decision is reached all the code is updated.

I think we should consider moving to "PrimaryEntrypointCommand" for a measure of consistency and code-self-explanation.

[DV8FromTheWorld]

Does renaming this break ABI? I haven't read further here, but this strikes me a possible problem

[freya022]

I believe yes, as the method signatures in the bytecode include the parameters and return type signatures

[freya022]

I think we should consider moving to "PrimaryEntrypointCommand" for a measure of consistency and code-self-explanation.

Slash command properties are also merged into Command, so I put the entrypoint stuff in it too, having a separate PrimaryEntrypointCommand subtype would require the user to check if it is an instance of such, while for slash commands it isn't needed at this moment. Are you sure you want to go with it?