Add some reasonable timeouts to API server

[nsmith5]

Summary

Adds a 1 minute hard coded read, header read, write and idle timeout. This helps prevents accidental or malicious connection exhaustion of the API server.

Implementation note

While simply setting ReadTimeout would have set the header read and write timeouts to the same value, I think setting them explicitly helps inform the future code readers about the server behaviour in case they're unfamiliar with the behaviour of http.Server timeouts.

Ticket Link

Fixes #336

Release Note

* Added 1 minute read, write and idle timeouts to API server

[haydentherapper]

Will this close all open connections, i.e. will this stop the connection to Trillian? Or should we also add a timeout for outbound connections?

[nsmith5]

This is just for inbound API requests. Timeouts on the outbound requests to the CT log are controllered here and it seems are set to 30 seconds https://github.com/sigstore/fulcio/blob/main/pkg/ctl/ctl.go#L37

[dlorenc]

LGTM but it hit a merge conflict with your other one!

[nsmith5]

Rebased on main and should be good to go now :)