Allow displaying of messages with custom mimetype data

[vojtamolda]

Hello everyone,

I have a proposal that allows people to have a simpler way of displaying messages with other mimetype than PNG.

The change allows the following example to work. Otherwise, it would be necessary to duplicate the entire JupyterDisplay.MessageContent struct and change the copy to work with HTML data:

%install '.package(url: "https://github.com/IBM-Swift/BlueCryptor.git", from: "1.0.0")' Cryptor
%include "EnableJupyterDisplay.swift"

struct HTMLData: Encodable {
    let html: String
    let text = "<IPython.core.display.HTML object>"
    private enum CodingKeys: String, CodingKey {
        case html = "text/html"
        case text = "text/plain"
    }
}

func display(html: String) {
    let htmlData = HTMLData(html: html)
    let data = JupyterDisplay.MessageContent(data: htmlData).json
    JupyterDisplay.messages.append(JupyterDisplay.Message(content: data))
}

The following cells can then output rich HTML representations:

let greenBox = "<div style='height: 100px; width: 100px; background-color: green'></div>"
display(html: greenBox)

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[vojtamolda]

@googlebot Signed!

[googlebot]

CLAs look good, thanks!

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