Proposal: Replace "behavioral" marker types with a tag

[flobernd]

We currently have some generic marker types defined which are used to provide "behavioral" context to the generators:

/**
 * Some APIs will return values such as numbers also as a string (notably epoch timestamps). This behavior
 * is used to capture this behavior while keeping the semantics of the field type.
 *
 * Depending on the target language, code generators can keep the union or remove it and leniently parse
 * strings to the target type.
 */
export type Stringified<T> = T | string

/**
 * A set of flags that can be represented as a single enum value or a set of values that are encoded
 * as a pipe-separated string
 *
 * Depending on the target language, code generators can use this hint to generate language specific
 * flags enum constructs and the corresponding (de-)serialization code.
 */
export type PipeSeparatedFlags<T> = T | string

Instead of modelling (de-)serialization behavior like this, we should introduce a proper tag to avoid unnecessary complexity (especially for languages that do not support generics).