Support allowEnumStringTypes in typescript-operations

**Is your feature request related to a problem? Please describe.**

I'm using `allowEnumStringTypes` option to be able to use the string representation of enums in my code, but this option is not handled by typescript-operations plugin, preventing me from using the string representation of enums as variables in my operations.

**Describe the solution you'd like**
`allowEnumStringTypes` should be handled by typescript-operations to be consistent with typescript plugin.

**Describe alternatives you've considered**
None

**Additional context**
Example of generated type:
```
export type GetTargetsQueryVariables = Exact<{
id: Scalars['String'];
type: TargetType;
}>;
```

Expected generated type:
```
export type GetTargetsQueryVariables = Exact<{
id: Scalars['String'];
type: TargetType | `${TargetType}`;
}>;
```