Cache content_sha256 field for Files API

Currently, we load the whole blob in memory on calling Files API endpoints. Loading a large file via Gitlab Rails may be severe from performance perspective.

We could avoid loading the whole blob and serve the request via Workhorse, like we plan to do in !67155 (merged), but we have X-Gitlab-Content-Sha256 header that is calculated based on the whole file content. The header has been added within gitlab-foss#47315 (closed) to verify the file content.

Caching the header using the blob-id will improve the performance

Related issue: #337308 (closed)