.Net: OpenAPI plugin & functions import filters

[SergeyMenshykh]

Consider having OpenAPI plugins filter and OpenAPI plugin functions filter that would be called at plugin import step. The filters may accept:

• Original OpenAPI document so the filter can access it if needed.
• Parsed model as a ref parameter. Calling code can access, modify and replace it if needed. Public OpenApi model surface should be revisited.
• Parser. Calling code may need to parse a subset of the original document.
• etc...

It should be possible:

• To select server url for an individual operation if more than one is provided.
• To select operation body schema if there's more than one - .Net: Support OpenAPI request body schemas represented by allOf, anyOf and 'oneOf' keywords #8997
• etc...