Enhancement: Add requestBody as option for customapi widget

[dstotz]

Proposed change

Add new optional requestBody option to customapi widget. This will allow accessing endpoints that might require filtering via request body instead of query params.

This new option will support objects or strings to allow for clean ways to write JSON request bodies or the ability to pass a raw body for non JSON formats.

Here is the services configuration I used:

- Custom API:
    - POST No Body:
        widget:
          type: customapi
          url: https://api.mockfly.dev/mocks/febdbed3-d77d-4665-b94f-d12ffb0813a9/test
          method: POST
          headers:
            Content-Type: application/json
            Accept: application/json
          mappings:
            - field: msg
              label: Message
              format: text

    - POST Object Body:
        widget:
          type: customapi
          url: https://api.mockfly.dev/mocks/febdbed3-d77d-4665-b94f-d12ffb0813a9/test
          method: POST
          headers:
            Content-Type: application/json
            Accept: application/json
          requestBody:
            foo: bar
          mappings:
            - field: msg
              label: Message
              format: text

    - POST String Body:
        widget:
          type: customapi
          url: https://api.mockfly.dev/mocks/febdbed3-d77d-4665-b94f-d12ffb0813a9/test
          method: POST
          headers:
            Content-Type: application/json
            Accept: application/json
          requestBody: "{\"foo\": \"bar\"}"
          mappings:
            - field: msg
              label: Message
              format: text

You can use the mock API I set up for testing if you would like: https://api.mockfly.dev/mocks/febdbed3-d77d-4665-b94f-d12ffb0813a9/test

It has a 500 request limit so don't leave it polling for too long or it will stop working. If it receives a POST with a request body containing {"foo":"bar"} it returns {"msg": "I am a POST with a body"}, otherwise it returns {"msg": "I am a POST with no body"}

Closes #2313
Closes #2102

Type of change

• New service widget
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation only
• Other (please explain)

Checklist:

• If applicable, I have added corresponding documentation changes.
• If applicable, I have reviewed the feature and / or service widget guidelines.
• I have checked that all code style checks pass using pre-commit hooks and linting checks.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.

[dstotz]

I updated the description as it seems like this change will also resolve #2102 as the only thing missing to use GraphQL API's was the ability to pass the request body with the query. I tested with one of my API's and can confirm it worked as expected.

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