dynamic_forward_proxy: fix sub_cluster_confg stuck with warm up due to DFPCluster removed

[itsLucario]

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Commit Message:

• A new attributeignore_removal has been added to cluster spec. This attribute will prevent the cluster manager from removing clusters with value set to true.
• A new function, removeClusterAddedViaApi, has been added. This function can be used by custom implementations to remove clusters and manage dynamic clusters on their own.

Additional Description:

• Currently clusters that are dynamically added with custom implementations from filters, etc. are removed when a CDS event is triggered. This is because these dynamically created clusters will come in diff of CDS and will be removed by the cluster manager.

Risk Level: low Testing: unit test Docs Changes: no Release Notes: no Platform Specific Features: [Optional Runtime guard:] [Optional Fixes commit #PR or SHA] [Optional Deprecated:] [Optional API Considerations:]

Fixes: #35171

[repokitteh-read-only[bot]]

Hi @itsLucario, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/35848 was opened by itsLucario.

see: more, trace.

[repokitteh-read-only[bot]]

As a reminder, PRs marked as draft will not be automatically assigned reviewers, or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/35848 was opened by itsLucario.

see: more, trace.

[repokitteh-read-only[bot]]

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/).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/35848 was opened by itsLucario.

see: more, trace.

[itsLucario]

@adisuissa Can you please help with review?

[ravenblackx]

Ping @adisuissa I think this is waiting for opinion from you.

[ravenblackx]

Pinged @adisuissa again, and also in slack.

[itsLucario]

/retest

[alyssawilk]

also this needs a main merge /wait

[repokitteh-read-only[bot]]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/35848 was synchronize by itsLucario.

see: more, trace.

[adisuissa]

I'm trying to rerun the job, but it may need another main merge (due to a recent CI upgrade)

[adisuissa]

Seems that CI complains about coverage. Please follow link at https://github.com/envoyproxy/envoy/actions/runs/11157572663/job/31012169240#step:12:6358 to see the code that is not covered. /wait

[itsLucario]

/retest

[itsLucario]

/retest