Provide string enum option

[rurutia]

Avro enums are only compiled to numeric enums

export enum colorTypes {
    red,
    blue,
    green,
}

It does not work for our scenario when having java consumer expecting string value from the same schema. It needs to be compiled to string enums like

export enum colorTypes {
    red = "red",
    blue = "blue",
    green = "green",
}

actually, I think by default typescript enum should always be constant-initialized with a string literal fpor avro serde as suggested by https://www.typescriptlang.org/docs/handbook/enums.html

String enums have the benefit that they “serialize” well. In other words, if you were debugging and had to read the runtime value of a numeric enum, the value is often opaque - it doesn’t convey any useful meaning on its own (though reverse mapping can often help), string enums allow you to give a meaningful and readable value when your code runs, independent of the name of the enum member itself.