Add "content-sha256" in File API

Description

Dear awesome community!

I'm currently using File API but there is no way to compare the downloaded sha256 file. I could download it, run sha256sum then go on gitlab server and run sha56sum on the local file.

(md5 is not secure, sha512 is probably overkill)

Proposal

Add content_sha256 (whatever)

GET

curl --request GET --header 'PRIVATE-TOKEN: mytoken' "https://mygitlab.com/api/v4/projects/myprojectid/repository/files/myfile?ref=master" | jq. 
{
  "file_name": "myfile",
  "file_path": "myfile",
  "size": 1639,
  "encoding": "base64",
  "content": "xxxxxxxxxxxxxxxxxx",
  "content_sha256": "e2104e9d433fdb416ac12931b6185c64a0c79784785815dc4bad26c40c40efc4",
  "ref": "master",
  "blob_id": "5bc3288f340f81215ce6b8a049ec14f8f6268193",
  "commit_id": "84061bbad73d0b3a2a598f43599355a4871034af",
  "last_commit_id": "78181ed46d6d8c489dcc96ec1fd5a8bb11a52a8d"
}

Links / references

aws s3: https://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonRequestHeaders.html