[FEATURE] Show upload form for extensions in Get Extensions section

This patch adds the upload form to the "Get Extensions" section in the
ExtensionManager.

Change-Id: Id8e698ba0d1c2cc1dddec61bba902d4913e1b01c
Resolves: #51776
Releases: 6.2
Reviewed-on: https://review.typo3.org/23639
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Markus Klein
Tested-by: Markus Klein