[FEATURE] ext:install Verify checksum of downloaded core

https://get.typo3.org/json provides checksums of the file content
of each TYPO3 CMS release. The expected sha1 is now compared with
the actual sha1 of the downloaded files to protect upgrades from
broken downloads and some attack vectors.

Change-Id: I1ec604ed2ef5f53abc930ff360ca2d7267e3c64d
Resolves: #52618
Releases: 6.2
Reviewed-on: https://review.typo3.org/24492
Reviewed-by: Sascha Egerer
Tested-by: Sascha Egerer
Reviewed-by: Markus Klein
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring