[Feature][TypeScript] request param enum as literal unions

[macjohnny]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

Allows request params of type enum to be represented as enum in the generated angular client code.

Before:

public findPetsByStatus(status: Array<string>, observe?: 'body', reportProgress?: boolean): Observable<Array<Pet>>;

after:

public findPetsByStatus(status: Array<'available' | 'pending' | 'sold'>, observe?: 'body', reportProgress?: boolean): Observable<Array<Pet>>;

fixes #7365

Replaces #7366

[macjohnny]

cc @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @wing328 @bedag-moo @JohannesHoppe

[macjohnny]

See also the discussion in #7366

[macjohnny]

it seems that this has already been broken before, see the previous lines:

 import {
   Array<string>,		  

[wing328]

@macjohnny what about fixing it by replacing {{ ... }} with {{{ ... }}} ?

[macjohnny]

@wing328 it is broken even if the html issue is fixed, since it tries to import Array<string> from the file, but this is a generic type...
I will open a separate issue to have this fixed.

[macjohnny]

see #7465

[macjohnny]

it seems this was introduced at some point before...

[macjohnny]

Could anyone of the technical comittee please revie this PR?
It is similar to #7366 and just requires your approval again.

[wing328]

I had a look at this PR and the change (which covers default codegen) looks good to me.

I'll merge this into master if no one has any question by coming Wednesday.

[svenhuber]

successfully tested for Angular