examples: Add custom load balancer example

[zasweq]

Contains #6679. This PR adds a custom load balancer example, deployed as the top level balancer of the channel through a service config. This custom load balancer is a petiole policy wrapping pick first, which is the intended way we want users to write their own custom load balancers, as users get features such as Sticky Transient Failure, Health Checking, and the Happy Eyeballs algorithm out of the box.

As part of this PR, pick_first is changed to work on the endpoints list and handle the endpoints list as defined in A61. This change works within system because the Channel is already emitting a Slice of Endpoints.

A helper interface between the petiole policy and the base pick first is also added. This lets petiole policy focus on the business logic, as seen by the simplicitly of custom_round_robin.

RELEASE NOTES:

• examples: Add custom lb example

[codecov]

Codecov Report

Merging #6691 (944499d) into master (adf976b) will decrease coverage by 0.47%.
Report is 6 commits behind head on master.
The diff coverage is 13.21%.

❗ Current head 944499d differs from pull request most recent head 7af9e93. Consider uploading reports for the commit 7af9e93 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6691      +/-   ##
==========================================
- Coverage   81.24%   80.77%   -0.47%     
==========================================
  Files         345      346       +1     
  Lines       33941    34094     +153     
==========================================
- Hits        27574    27541      -33     
- Misses       5202     5379     +177     
- Partials     1165     1174       +9     

Files	Coverage Δ
pickfirst.go	82.69% <88.46%> (+0.47%)	⬆️
balancer/balanceraggregator/balanceraggregator.go	0.00% <0.00%> (ø)

... and 24 files with indirect coverage changes

[ginayeh]

Please rebase your comment before merging.

[zasweq]

Done.

[dfawley]

I have a lot of comments. Maybe we should just simplify this for now and make something small that doesn't need to worry about being a petiole policy just so we can get it done with a reasonable amount of effort.

[zasweq]

Got to all comments. Thanks for the pass.

[zasweq]

Added BalancerAggregator utility we discussed offline :). This is ready to review now.

[dfawley]

Can you please add a simple test or 2 into the endpointsharding (or whatever we call it) package, please?

[zasweq]

Added e2e test that tested the main behaviors I view endpoint sharding to have (a child per endpoint, default behavior of round robin over pickers in aggregated state, access to child picker in parent UpdateState).

[dfawley]

🎉

Now to rewrite round_robin 😆

[zasweq]

And move pick first :).