cmd/gitserver/server: Extract arg to conf.Cached into a func for testability

Created by: indradhanush

Making this into a function allows us to test the code to guarantee that multiple certificates configured in the config are written into the SSLCAInfo file correctly.

Also we don't need the type hint anymore since the type is now inferred from the return type of the function itself. Which makes the code easier to read.

Change inspiration out of looking into https://github.com/sourcegraph/customer/issues/1136 and https://github.com/sourcegraph/sourcegraph/issues/38128 respectively this week.

Test plan

Added a test case.