go: Fix missing imports for optional body params.

[booxter]

Previous mustache template was using #isBodyParam outside of #operation
context, so it was not effective.

Even if we'd add the proper context, we'd then risk generating duplicate
imports for multiple matching parameters.

For this reason, this patch implements detection of an optional body
parameter in code, making sure the corresponding import is added just
once.

Fixes #19237

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[booxter]

@antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5

[wing328]

tested the fix locally and the result is good

thanks for the contribution.

[wing328]

FYI. Merged #22034 to add a test covering this issue moving forward