Feature: #98914 - TypoScript as request attribute

Description

The TYPO3 frontend middleware chain now sets up the request attribute frontend.typoscript. This is an instance of \TYPO3\CMS\Core\TypoScript\FrontendTypoScript and contains the calculated TypoScript settings (formerly "constants") and sometimes setup, depending on page cache status.

When a content object or plugin (plugins are content objects as well) needs the current TypoScript, it can retrieve it using this API:

// New substitution of $GLOBALS['TSFE']->tmpl->setup
$frontendTypoScriptSetupArray = $request->getAttribute('frontend.typoscript')->getSetupArray();

The FrontendTypoScript attribute contains some more getters:

getSettingsTree(): The TypoScript settings as object tree. This tree is still a bit experimental in TYPO3 v12 and should only be used if really needed for now, it may still change. It is marked internal at the moment.

The constants tree is always set up in frontend requests: It is needed early for page cache determination, content objects can expect it to be set.
getFlatSettings(): The TypoScript settings as flat array. Example TypoScript:
```
mySettings {
    foo = fooValue
    bar = barValue
}
```
Copied!
Result array:
```
$flatSettings = [
    'mySettings.foo' => 'fooValue',
    'mySettings.bar' => 'barValue',
];
```
Copied!
The settings array is always set up in frontend requests: It is needed early for page cache determination, content objects can expect it to be set.
getSetupTree(): The TypoScript setup as object tree. This tree is still a bit experimental in TYPO3 v12 and should only be used if really needed for now, it may still change. It is marked internal at the moment.

The setup tree is only set up, if a frontend request could not be satisfied from page cache and a full page content calculation is required, or if a page cache does exist, but contains USER_INT or COA_INT that have to be calculated for each call. Effectively, when a content object rendering is called, the object can expect the setup object tree to be set.
getSetupArray(): An array representation of the setup tree. This is identical to the old TYPO3\CMS\Core\TypoScript\TemplateService->setup that was usually accessed using $GLOBALS['TSFE']->tmpl->setup.

This is the main API to retrieve frontend TypoScript for now. Content objects do receive the current request from the rendering chain and can retrieve the full TypoScript this way, if needed. Note that content objects also retrieve the "local" content object configuration already, an access to the full TypoScript in general is only needed in seldom cases.

The setup array is only set up if a frontend request could not be satisfied from page cache and a full page content calculation is required, or if a page cache does exist but contains USER_INT or COA_INT that have to be calculated for each call. Effectively, when a content object rendering is called, the object can expect the setup object tree to be set.

Impact

This is a substitution especially of the deprecated \TYPO3\CMS\Core\TypoScript\TemplateService, typical old calls were TypoScriptFrontendController->tmpl or $GLOBALS['TSFE']->tmpl, often reading the setup property using tmpl->setup to grab the current TypoScript setup array. These calls should be avoided, the TemplateService and the tmpl property will be removed in TYPO3 v13.