Default Content-Type: application/json for REST and GraphQL Actions

Context:
Users are attempting to send form data from Clutch to external services like Directus, which expect application/json payloads. However, Clutch's current default behavior sends the body as plain text unless the header is manually set.

Problem:

• REST/GraphQL actions do not default to sending Content-Type: application/json, resulting in empty or invalid request bodies unless users manually set the header.

• This leads to confusion for users unfamiliar with header configuration, especially when trying to integrate with common services like Directus or WordPress.

• Current workaround: users must manually add the header via the action config.

Proposed Solution:

• Default Content-Type: application/json if no header is set by the user.

• Allow header override: if a user explicitly defines Content-Type, respect their value.

• Consider exposing a dropdown UI for commonly used headers like Content-Type: application/json, Authorization, etc., to reduce manual setup and improve UX.

Design Suggestions:

• UX reference: Strapi's GraphQL playground or Postman-style dropdowns.

• Could be part of the REST/GraphQL action panel or a reusable presets module.