refactor!: make Webhook event type an enum

[oana-lolea]

Changes proposed in this pull request

• Added WebhookEventType enum that contains all the possible webhook event types
• Added utility enums that contain members of the WebhookEventType enum (for incoming payments, wallet address etc.)

Context

I created WebhookEventType enum that contains all the possible webhook event types. This changes the type format to the standard enum format (uppercase words separated by underscore, e.g. 'INCOMING_PAYMENT_CREATED'), therefore a migration was needed to update the existing types.
Due to this change, I added subsets of this enum for classes that extend WebhookEvent.
Tests were also updated using the new enum.

Fixes #2786

Checklist

• Related issues linked using fixes #number
• Tests added/updated
• Make sure that all checks pass
• Bruno collection updated (if necessary)
• Documentation issue created with user-docs label (if necessary)
• OpenAPI specs updated (if necessary)

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