Add missing webhook thread endpoints and allow overriding webhook message attachments

[merlinfuchs]

This PR adds the missing attachments field to the WebhookEdit struct so you can remove and override attachments on webhook messages. See https://discord.com/developers/docs/resources/webhook#edit-webhook-message

It also adds the missing thread specific equivalents to the WebhookMessage, WebhookMessageEdit, and WebhookMessageDelete endpoints.

[FedorLap2006]

Thank you for implementing these missing features, however, I think it's a good idea to separate the endpoint and attachment changes into different PRs, since they have completely different scopes.

[merlinfuchs]

#1392 adds the attachments parameter so I will remove it from this PR