cliconfig: Registry protocol retry count and timeout settings

These were previously settable only via environment variables. These are
now handled as part of CLI Configuration and so also settable in a new
"registry_protocols" block in a CLI configuration file, with the
environment variables now treated as if they are an additional virtual
configuration file containing the corresponding settings.

This handles our settings in our modern style where package cliconfig is
responsible for deciding the configuration and then package main reacts
to that configuration without being aware of how it is decided.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/command/cliconfig/testdata/registry-protocols-only-timeout

@@ -0,0 +1,3 @@
+registry_protocols {
+  request_timeout_seconds = 50
+}