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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