openapi: fix missing request schema for custom plugins

[berkitamas]

Description

This pull request resolves #30714.

If a custom plugin is added to Vault, OpenAPI Explorer does not show properly the Request body for the plugin's POST endpoints. It shows a "string" instead of filling out the parameters based on the path fields.

github.com/mitchellh/mapstructure does a strings.EqualFold check between a struct key and a map key. This does not work for $ref key, a custom name match is needed to cover this edge case.

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