Refactor model dropdown footer

What does this MR do and why?

Addresses review feedback from: Ensure instance-level model selection page is a... (!208017 - merged), specifically:

• !208017 (comment 2811166293): instead of renaming the prop, I opted to refactor the ModelSelectDropdown component to render a footer slot and move the rendering of the the self-hosted button to ModelSelector component used in the self-hosted page. I also refactored the header text to an optional prop.

References

Follow-up from "Ensure instance-level model sel... (#576115)

Screenshots or screen recordings

Namespace model selection page	Duo Self-Hosted page	Add self-hosted model page

How to set up and validate locally

For Duo Self-Hosted pages:

• Run GDK on non-SASS mode: GITLAB_SIMULATE_SAAS=0 gdk start
• Run the following setup script: GITLAB_SIMULATE_SAAS=0 bundle exec 'rake gitlab:duo:setup'
• Duo Self-Hosted page:
  • Go to /admin/gitlab_duo/self_hosted
  • Click on model dropdown input, note that the footer section is rendered (Add self-hosted model button).
• Add Self-Hosted model page:
  • Go to /admin/gitlab_duo/self_hosted/models/new
  • Click on model dropdown, note no footer section rendered.

For Namespace model selection page:

• Run GDK on SASS mode: GITLAB_SIMULATE_SAAS=1 gdk start
• Run the following setup script: GITLAB_SIMULATE_SAAS=1 bundle exec 'rake gitlab:duo:setup'
• Go to /groups/gitlab-duo/-/settings/gitlab_duo/model_selection
• Click on model dropdown input, note that no footer section rendered.

MR acceptance checklist

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