Add a generic mechanism to add extension to node that don't have a typespec type equivalent

[skywing918]

There is many nodes in the openapi document where you can add extension but they don't have a typespec type you can use @extension to.
This include:

• servers
• paths
• requestBody
• responses object
• response object
• body
• externalDocs
• OAuth Flows Object
• OAuth Flow Object

[timotheeguerin]

For parameters i don't think this is something we'll want to support as the goal was to provide some common params which is achieved directly in TypeSpec and duplicating those in each operation is equivalent.

For summary and description I also wonder what the usefulness of this is and feel like this is something that need user request to consider.

[markcowl]

• consider adding a general mechanism for providing values for fields in OpenAPI that are not directly supported by the emitter