Adds output type=local support

[njlr]

This PR adds support for output type=local.

This is useful when the desired output from BuildX is a file, not an OCI image.

Whilst it would be possible to build an OCI image, flatten the layers and pick out the desired files, this way is probably more efficient.

I would appreciate some feedback on the UX and can make changes as needed. Thanks!

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Test plan

• New test cases added

[CLAassistant]

All committers have signed the CLA.