Add htmlNormalization event to ClipboardPipeline.

[Mati365]

Suggested merge commit message (convention)

Feature (clipboard): It's now possible to attach custom HTML serialize functions to clipboard pipeline, just before processing it into view fragment. Closes #17309
Internal (paste-from-office): Use inputHtmlNormalization clipboard event to handle HTML normalization.

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Additional information

The ClipboardPipeline is performing some HTML normalization (on string) just before converting it to the view document fragment:

ckeditor5/packages/ckeditor5-clipboard/src/clipboardpipeline.ts

Lines 229 to 233 in d874050

	if ( dataTransfer.getData( 'text/html' ) ) {
	contentData = normalizeClipboardHtml( dataTransfer.getData( 'text/html' ) );
	} else if ( dataTransfer.getData( 'text/plain' ) ) {
	contentData = plainTextToHtml( dataTransfer.getData( 'text/plain' ) );
	}

• There is no easy extension point to add additional normalizers.
• The PasteFromOffice is ignoring the prepared view document fragment and is performing a separate normalization on the data transfer payload:
  

  ckeditor5/packages/ckeditor5-paste-from-office/src/pastefromoffice.ts

  Line 83 in d874050

  const htmlString = data.dataTransfer.getData( 'text/html' );

  
  

  ckeditor5/packages/ckeditor5-paste-from-office/src/pastefromoffice.ts

  Line 88 in d874050

  data._parsedData = parseHtml( htmlString, viewDocument.stylesProcessor );

  
  Those cases could be unified as an extension point in the ClipboardPipeline so non-parsed data could be processed before conversion to the view document fragment.

~Kuba

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⚠️ I'm not pretty sure if this flag is still needed: 79555ac#diff-4b980c1afbffe5148af9316676ffda4ac77a98664dc69fcf7c07ba7726d3c42cR47. It's not fully documented so I kept it in evt.return property.