fix(cleanup): change the naming to be endpointpickerref

[capri-xiyue]

What type of PR is this?

What this PR does / why we need it:
Rename the filed to make it more clear

Which issue(s) this PR fixes:

part of #1173
Fixes: #1326

Does this PR introduce a user-facing change?:

Users need to change the` extensionRef` to  `endpointPickerRef`

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[capri-xiyue]

/assign @kfswain

[danehans]

xref #1421 for API docs.

[capri-xiyue]

xref #1421 for API docs.

Will do it in another PR as it is not directly related to this PR

[danehans]

Will do it in another PR as it is not directly related to this PR

@kfswain will be pushing a fix.

[kfswain]

/approve

This looks good but would like to let: #1422 merge before we merge this, in case we want to update the docs

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: capri-xiyue, kfswain

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[danehans]

/lgtm