async/promises are unable to resolve in ETA templates

Hi!

Love your package, very useful. We were attempting to use custom ETA templates but we noticed promises don't appear to be supported currently by `swagger-typescript-api`.

# Minimal replicable example

**templates/api.eta**

```
<%
const test = Promise.resolve('test');
%>

<%~ await test %>
```

**openapi.json**

Copied from https://raw.githubusercontent.com/OAI/OpenAPI-Specification/main/examples/v2.0/json/petstore.json (doesn't really matter what the contents of this file are in this case)

**Command to run**

`yarn swagger-typescript-api --templates ./templates -p ./openapi.json`

## The error

```
(node:35256) UnhandledPromiseRejectionWarning: Error: Swagger schema parse error!
Eta Error: Bad template syntax

await is only valid in async function
```

# Why would this be useful?

Our current use case is to allow the ability to use `import` syntax. You can technically use `import` already like so:

```
<%
import('micromatch').then(console.log);
%>
```

but it's not useful without being able to, ultimately, resolve the promise as mentioned previously. Another alternative would be to add support for `require` but I think supporting promises would be more useful as it would enable anything that returns a promise to be used (E.g. `readFile` or `fetch`), not just `import`.

# Potential solution

I believe all that needs to be done in order to support this feature is to make the functions that render the template internally within swagger-typescript-api `async`.

I'm quite happy to add this feature myself if you agree with the solution.

For instance:

Would be come: