Support string enums as keys in maps

## :rocket: Feature Request

### Affected Languages

- [X] `TypeScript` or `Javascript`
- [X] `Python`
- [X] `Java`
- [X] .NET (`C#`, `F#`, ...)

### General Information
* **JSII Version:** 1.7.0 (build 179a3a5), typescript 3.9.5
* **Platform:** Darwin $HOST 19.5.0 Darwin Kernel Version 19.5.0: Tue May 26 20:41:44 PDT 2020; root:xnu-6153.121.2~2/RELEASE_X86_64 x86_64

* [ ] I may be able to implement this feature request

* [ ] This feature might incur a breaking change

### Description

Right now jsii only allows maps with string key. If instead a enum with strings is used the compile fails:

```
export enum CspDirectiveName {
CONNECT_SRC = "connect-src",
}

const cspDirectives: { [P in CspDirectiveName]?: string } = {};
// alternative
const cspDirectives: Partial> = { [CspDirectiveName.CONNECT_SRC] = "asdf"};

cspDirectives[CspDirectiveName.CONNECT_SRC] = "https://amazon.com"
```

error

```
error TS9999: JSII: Only string-indexed map types are supported

152 readonly cspDirectives: { [P in CspDirectiveName]?: string[] };
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

### Proposed Solution

I'm actually not sure if that is really a jsii problem or more a problem in TypeScript which supports enum keys only with computed types. But I think it's a valid use-case for jsii as well.

Reproduce the TypeScript enum-keyed map example against jsii 1.7.0 and confirm the TS9999 error. Trace the compiler/type-validation entry point that rejects non-string-indexed maps, then define tests showing that string enum keys compile successfully and preserve the requested map behavior.

Written by the indexing model from the issue text.