[FEATURE] Introduce backend layout data providers

Backend layouts are currently stored in the database as
regular records. Since extension cannot ship their specific
backend layout definitions, data providers become handy in
providing the accordant information taken from e.g. static
files in the file system.

Data providers can be registered like shown in the follow and
need to be implement DataProviderInterface of the namespace
TYPO3\CMS\Backend\View\BackendLayout:

$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']
	['BackendLayoutDataProvider'][$_EXTKEY] = 'Classname';

A DataProviderContext object is used to transport submitted
data (e.g. table or field name) to the accordant data providers.

Change-Id: I2e3d39e720c6d1bffa9a586148f0eea4cab0210e
Resolves: #37208
Documentation: #52809
Releases: 6.2
Reviewed-on: https://review.typo3.org/11804
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader