Implement ImageProcessingMode which dictates how images should be handled during conversion

[DanAtkinson]

Hello

The idea of this PR is to provide a working example of being able to dictate how HtmlToOpenXml should handle images that are.

This is done by overriding ImageProcessing in the HtmlConverter constructor thus:

HtmlConverter htmlConverter = new(mainPart, new DefaultWebRequest())
{
    // Default value of ImageProcessingMode is ImageProcessingMode.Embed (the current behaviour).
    ImageProcessing = ImageProcessingMode.LinkExternal
};

I've also provided handling for data uris but my primary focus here has been to swap external images out so that they use Blip.Link instead of Blip.Embed. As stated above, the default behaviour remains the same (e.g. ImageProcessing.Embed) unless explicitly overridden.

Example implementation - taken from a modified form of examples\Demo\Program.cs

string inputFile = "C:\\ConvertedHtml.html";
string htmlContent = await File.ReadAllTextAsync(inputFile);

string outputPath = "C:\\ConvertedHtml.docx";
using WordprocessingDocument wordDoc = WordprocessingDocument.Create(outputPath, WordprocessingDocumentType.Document);
MainDocumentPart mainPart = wordDoc.AddMainDocumentPart();
mainPart.Document = new Document(new DocumentFormat.OpenXml.Wordprocessing.Body());

HtmlConverter htmlConverter = new(mainPart, new DefaultWebRequest())
{
    ImageProcessing = ImageProcessingMode.LinkExternal
};

await htmlConverter.ParseBody(htmlContent);

AssertThatOpenXmlDocumentIsValid(wordDoc);

mainPart.Document.Save();

During my tests, I have found that a Word document with 30 embedded external images has been reduced from 4MB to 10KB, so there is a sizeable reduction.

Further changes could be made later, for example providing additional rules for dictating which domains should/shouldn't be embedded, or perhaps providing a mechanism for HTML-level customisation of image processing (for example by use of an attribute such as <img src="..." data-imageProcessing="Embed" />

I also added 5 new unit tests (in ImgTests.cs) to support my changes and made some highlighted spelling corrections to some usages of asynchronous and intended. My apologies if some changes to formatting occurred. I use ReSharper and my Visual Studio formats the documents such as braces, imports, access modifiers, etc, and I've done my best to discard those changes in my PR.

Kind regards
Dan Atkinson - Atcore Technology Ltd

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[onizet]

This is a delightful PR! Complete, with test cases, documented. That's very nice, thank you very much.
There is some small improvement, like src is not used inside ImageExpression.CreateBlip().
Or asserting that the HttpMock hasn't been called with LinkExternal.

By experience, I know that when I request some change in a PR, I never see back the author. So I'm going to merge this PR and do those minor changes myself.
Thank you for your valuable support.