Gitaly: Extend timeout for blob fetching

What does this MR do and why?

Fetching very large blobs can cause GRPC timeout errors. Extend the timeout.

Fixes: #342703 (closed)

How to set up and validate locally

1. Create a repository with a very large (7GB on my machine) file
2. View the commit that adds the file
3. In the old version, this causes a 500 error, but in the new version, it works properly

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