OpenAPI: make servers and security schemes configurable/overridable

[g-linville]

It would be helpful for users to be able to override the security schemes and servers defined in an OpenAPI file when they reference it in the tool.

For example, the GitLab OpenAPI file points to the main, public GitLab instance, but it is common for companies to self-host it. It would be helpful for people to be able to refer to our main GitLab tool without having to clone it and modify the server specified in the file.

It would also be helpful in some cases to choose between multiple existing security schemes in an OpenAPI file, so they can choose which one GPTScript generates credential references for in the resulting tools.

I have no idea what the syntax for this would look like, but it is something to consider.