cluster specifier: support new attempt aware cluster specifier

[wbpcode]

Commit Message: cluster specifier: support new attempt aware cluster specifier
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[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @adisuissa
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #46640 was opened by wbpcode.

see: more, trace.

[wbpcode]

The metadata self is extendable and it should be easy to support clusters there if we want per-request level fallback order and traffic distribution.

[wbpcode]

For example, the default order and configuration:

  priority_groups:
  - name: local
    - name: a
      weight: 20
    - name: b
      weight: 80
  - name: remote
    clusters:
    - name: c
      weight: 20
    - name: d
      weight: 80

the metadata could be:

envoy.my_filter:
  priority_groups:
  - name: remote
    clusters:
    - name: c
      weight: 80
    - name: d
      weight: 20
  - name: local

Then, the order will be overridden. And for the specific request, the group remote's clusters traffic distribution also will be overridden, but the group local, it's will use the default distribution.