Feature: #101391 - Add base64 attribute to ImageViewHelper

See forge#101391

Description

The ViewHelpers <f:image> and <f:uri.image> now support the attribute base64="true" that will provide a possibility to return the value of the image's src attribute encoded in base64.

<f:image base64="true" src="EXT:backend/Resources/Public/Images/typo3_logo_orange.svg" height="20" class="pr-2" />
<img src="{f:uri.image(base64: 'true', src:'EXT:backend/Resources/Public/Images/typo3_logo_orange.svg')}">
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Will result in the according HTML tag providing the image encoded in base64.

<img class="pr-2" src="data:image/svg+xml;base64,PHN2...cuODQ4LTYuNzU3Ii8+Cjwvc3ZnPgo=" alt="" width="20" height="20">
<img src="data:image/svg+xml;base64,PHN2...cuODQ4LTYuNzU3Ii8+Cjwvc3ZnPgo=">
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This can be particularly useful inside \TYPO3\CMS\Core\Mail\FluidEmail or to prevent unneeded HTTP calls.