fix schema registry proxy handling

[jsvd]

change schema_registry_proxy setting from uri to string to move URI validation further down so that "" is an acceptable value. accepting "" is useful when doing something like:

  schema_registry_proxy => "%{SCHEMA_REGISTRY_PROXY}"

But depending on environment that env var may be empty. This removes the need for templating.

To support this we need to validate the URI a bit later.

Also this PR removes uri handling of https and basic auth for the proxy url given that the schema registry itself doesn't support it either (see confluentinc/schema-registry#2137)

fixes #144