API: Backend

[guydc]

What this PR does / why we need it:

This is a preliminary API proposal for discussion purposes only. The new Backend API allows users to reference UDS, FQDN and IP backends with an alternative to the K8s Service resources. See design doc for more details.

Which issue(s) this PR fixes:
Relates to #2997, #36

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.14%. Comparing base (c4b0216) to head (5b69c70).
Report is 1 commits behind head on main.

❗ Current head 5b69c70 differs from pull request most recent head 3148915. Consider uploading reports for the commit 3148915 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3063      +/-   ##
==========================================
+ Coverage   66.97%   67.14%   +0.17%     
==========================================
  Files         164      165       +1     
  Lines       23882    23845      -37     
==========================================
+ Hits        15994    16011      +17     
+ Misses       6964     6916      -48     
+ Partials      924      918       -6     

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[zirain]

so these's something like ServiceEntry in Istio?

[Xunzhuo]

I like this idea, Is this like istio`s ServiceEntry ?

[guydc]

@zirain, @Xunzhuo - There are some differences from ServiceEntry.

• ServiceEntry doesn't support Unix Domain Sockets
• You can route from VirtualService to external services by just specifying their address. You don't need a ServiceEntry for that, but a ServiceEntry allows you to configure some setting related to DNS resolution, (m)TLS, etc. In EG, this resource (or Service + FQDN Endpoint) is mandatory for external routing.

[guydc]

/retest

[guydc]

/retest

[arkodg]

LGTM thanks ! looking forward to using this API

[zhaohuabing]

LGTM thanks!