add support for commands and interactions

[switchupcb]

Adds tested support for slash commands.

Addresses: #11, #31

• Security and Authorization (Moved to next release)

Application Command Examples

• Basic Command
• Subcommands
• Options
• Localization
• Autocomplete
• Interaction Response (Basic)
• Edit Interaction Response
• Follow-up Message Response
• Edit Follow-up Message Response

Tools

• InterceptSIGINT: Program waits until termination from terminal.

[switchupcb]

The following articles were relevant to creating a working unmarshal.go implementation.

• https://stackoverflow.com/questions/53989758/unmarshal-embedded-field-pointer-panics-when-field-implements-unmarshaljson
• encoding/json: panic on embedded struct pointer that implements Unmarshaler golang/go#30148
• https://eli.thegreenplace.net/2020/embedding-in-go-part-1-structs-in-structs

[switchupcb]

Based on the information provided in Interactions: Security and Authorization verification only applies to bots that receive interactions via outgoing webhook. The criteria for an outgoing webhook is defined here. Currently, Disgo supports the first method of the two mutually exclusive methods to receive interactions.

Implementing outgoing webhooks requires three steps.

1. The user must set the "Interactions Endpoint URL" setting in the Developer Portal.
2. That endpoint must be prepared to ACK a PING message. It's unclear whether Disgo should provide support for this directly (through the API Wrapper) given the following.
   • There is category of interaction-webhook libraries such that this logic may indicate more than the implementation of all requests (against).
   • All webhook requests are already implemented in Disgo (for).
   • Based on Tempest, all that would be needed is a server which is less complexity than the implemented Discord Gateway (neutral).
   • The Discord Gateway allows Disgo users to respond to interactions already (against).
3. The endpoint must handle signature headers upon each interaction.

Implementation

Implementation for this feature is straightforward.

1. Add a PublicKey field to client.
2. Determine whether webhooks interactions are bound by rate limits. If not, add those endpoints to the IgnoreGlobalRateLimitRouteIDs map in ratelimit.go.
3. Add a server to webhook.go which receives requests and verifies the headers of incoming requests.
4. Add tests and examples.

In any case, that feature is currently beyond the scope of this pull request such that its target will be moved to the next release.