### What changes were proposed in this pull request?
`./build/mvn` now downloads the .sha512 checksum of Maven artifacts it downloads, and checks the checksum after download.
### Why are the changes needed?
This ensures the integrity of the Maven artifact during a user's build, which may come from several non-ASF mirrors.
### Does this PR introduce _any_ user-facing change?
Should not affect anything about Spark per se, just the build.
### How was this patch tested?
Manual testing wherein I forced Maven/Scala download, verified checksums are downloaded and checked, and verified it fails on error with a corrupted checksum.
Closes#32505 from srowen/SPARK-35373.
Authored-by: Sean Owen <srowen@gmail.com>
Signed-off-by: Sean Owen <srowen@gmail.com>
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