Support setting fallback endpoints in EPP picking

[ahg-g]

What would you like to be added:

Amend the endpoint picker protocol to define how EPP can respond to the proxy not only with a target endpoint, but also with fallback options.

Why is this needed:

To support retries.

[ahg-g]

/assign @LiorLieberman

[danehans]

xref: envoyproxy/envoy#38757

[liu-cong]

@LiorLieberman

This issue is currently in the v0.4 release tracker and v0.4 release is planned for Mid-June. Can you provide an estimate if this can be finished in the week of June 9?

Based on the estimate, we can decide whether to include this or postpone it to the v0.5 release. Thanks!

[ahg-g]

This will be easy to do once we implement the full pluggable scheduling proposal.

[ahg-g]

/unassign @LiorLieberman

[nirrozenbaum]

/assign