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Allow to pass options to the underlying diagram libraries

See original GitHub issue

Some diagram libraries relies on options declared outside of the diagram definition. These options are usually used to configure the output: margin, padding, size, style, font, layout…

For instance, when using Mermaid, we can define a configuration object to configure the font size, the layout direction or the maxium width and height: https://mermaid-js.github.io/mermaid/#/./Setup?id=configuration

The goal of this issue is to define a generic format to pass these options using the API. Then, in a seconde phase, we will progressively add support for options on each diagram libraries (potentially disabling options that might be harmful in safe mode).

Query parameters

For GET requests, it makes sense to use query parameters:

GET /graphviz/svg/eNpLyUwvSizIUHBXqPZIzcnJ17ULzy_KSanlAgB1EAjQ?key=value

POST requests

POST /

{
  "diagram_source": "digraph G {Hello->World}",
  "diagram_type": "graphviz",
  "output_format": "svg",
  "diagram_options": {
    "key": "value"
  }
}

If the same option is defined as both a query parameter and as an attribute in the JSON body, I think the option defined in the JSON body should have an higher precedence.

HTTP headers

Since we can also send diagrams as plain text, we should also support HTTP headers:

POST /graphviz

Accept: image/svg+xml
Content-Type: text/plain
Kroki-Diagram-Options-Key: value

digraph G {
  Hello->World
}

In this case, it’s important to use a prefix to disambiguate option names from other header names. Again, if the same option is defined as both a query parameter and as a request header, I think the option defined as a request header should have an higher precedence.