Legacy RLS: fix Envoy build to pickup proper proto file from service/…

…ratelimit/v1.

This commit fixes the build failure when Datawire patch for legacy
ratelimit service is applied.

With this change Envoy would continue making RateLimitService method
call to pb.lyft.ratelimit path. So clients who have their own
RateLimitService based on lyft rls service would be able to upgrade to
Envoy 1.13 without any issue.

Note that I tried to use tools/proto_format for generating v1 things
under generated_api_shadow but it seems it's not suited for downgrading
rather it's useful for migration from v2 to v3 and probably to v4 for
future. So I manually place the legacy files under generated_api_shadow.