334 fix endpoint validation

[mmacai]

Description

Closes #334

• fixed proxy endpoint validation to not require path when path_regex is set

What to look out for

Dear reviewer, I want you to

• gain knowledge - I think it's important for you to know about the change.
• check that the code works on your machine.
• suggest implementation-code design/structure/readability improvements.
• suggest test-code design/structure/readability improvements.
• suggest documentation improvements.
• suggest language/writing improvements.
• help to find bugs (described below!).

You can try to run RIG with this (on this branch vs master branch):

PROXY_CONFIG_FILE='[{"id":"my-service","name":"my-service","version_data":{"default":{"endpoints":[{"id":"my-endpoint","method":"GET","path":"/","path_replacement":"/different-endpoint"},{"id":"my-transformed-endpoint","method":"GET","path_regex":"/foo/([^/]+)/bar/([^/]+)","path_replacement":"/bar/\\1/foo/\\2"}]}},"proxy":{"use_env":true,"target_url":"API_HOST","port":7070}}]' \
mix phx.server

Process

The goal is to improve not only the code in this PR but also our skills! The "rules":

• The review is considered "done" as soon as all reviewers have added their review, and all their comments have been addressed.
• For knowledge-sharing reviews, each reviewer should "approve" the PR after studying its content.
• After the approval, the merge is concluded by the developer.

Have fun!

[mmacai]

@kevinbader Updated it in a way that it should be more clear and also catch the use-case when multiple path types are set.