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attach: Move the AttachFunc default function to the initializer
Fixes a partially constructed AttachOptions
**What this PR does / why we need it**: NewAttachOptions partially constructs an AttachOptions structure. The defaultAttachFunc should be set automatically, so the caller can potentially override the default behavior.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
/cc @kubernetes/sig-cli-api-reviews