feat(js): Cast tests with EchoResponse, improve JS template

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🧭 What and Why

🎟 JIRA Ticket: https://algolia.atlassian.net/browse/APIC-250

Changes included:

Lots of changes, but it's mostly auto generated code, don't worry!

• Provide EchoResponse type which allow casting the response in our CTS and remove use of the as any casts.
  • I was initially going to also test the type of the parameters, but due to serialization of the payload it adds a lot of logic for a not-that-necessary feature.
• Remove any use of date-time in our specs
  • There's a lot of different date format handled in the REST API, which creates conditional templates, that are in the end stringified.
• Remove use of the model/models.ts file, we only authenticate in headers so we can have a simple function in the class.
• Fix response in the abtesting client (sorry it's out of the scope of this PR)

🧪 Test

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I still feel like having the method params generic would be better than casting, but I don't really want to impact the API for our own testing strategy

[millotp]

Looking really good !

[millotp]

Magnificent