Allow creation of multiple client instances

[bchrobot]

I could not find a TypeScript codegen library that fit the following criteria:

• Worked with the Telnyx Swagger document. A few threw parsing errors despite claiming to support OpenAPI v3 and the Telnyx spec passing an OpenAPI v3 validator.
• Is JS-based. We want a simple dev environment.
• Supports multiple clients. We need to be able to support interacting with multiple Telnyx backends from the same Node process; a singleton client won't do.

Overview of the issues in the space (2018): https://biercoff.com/my-small-investigation-about-swagger-codegen-generation-in-typescript/

• swagger-axios-codegen. This seemed promising but was unable to parse the spec (error Could not find property type on schema).
• swagger-typescript-codegen. Seems overly complicated. Only supports v2 (swagger) and not v3.
• @openapitools/openapi-generator-cli. Java-based, which is not ideal. File-based configuration did not work as expected.
• @devexperts/swagger-codegen-ts. Had trouble parsing the spec. Complained about schema: {}, among other things.
• openapi-typescript-codegen. This is what we are currently using, but authorization is configured on singleton (see Suggest to remove singleton OpenAPI object and make Clients instanced. ferdikoomen/openapi-typescript-codegen#703).
• @propps/openapi-typescript-codegen -- A fork of our current choice that is meant to address global configuration issue! It does not work...

[bchrobot]

This is now possible with openapi-typescript-codegen.