Add TypeMarker static factory method

[pkoenig10]

There are a variety of use cases where consumers may want to derive one TypeMarker into another.

For example, if you want to delegate to the default JSON serializer but add some additional serialization logic:

interface WithHeaders<T> {
    // Some JSON value
    T body();

    Map<HttpString, String> headers();
}

class MyResponseSerializerFactory<T> implements SerializerFactory<MyResponse<T>> {
    public <R extends WithHeaders<T>> Serializer<R> serializer(TypeMarker<R> type, UndertowRuntime runtime, Endpoint endpoint) {
        // I can't create a anonymous TypeMarker here,
        // but I can obtain a Type from the provided TypeMarker
        Serializer<T> bodySerializer = runtime.bodySerDe().serializer(/* TypeMarker */);

        return (value, exchange) -> {
            response.headers().forEach(exchange.headers()::put);
            bodySerializer.serialize(value.body(), exchange);
        };
    }
}

This PR adds a TypeMarker.of(Type) method to support these use cases.

This method is completely unchecked. Callers are responsible for choosing the correct type parameter.

[changelog-app[bot]]

Generate changelog in changelog-dir>`changelog/@unreleased`</changelog-dir

What do the change types mean?

- `feature`: A new feature of the service. - `improvement`: An incremental improvement in the functionality or operation of the service. - `fix`: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way. - `break`: Has the potential to break consumers of this service's API, inclusive of both Palantir services and external consumers of the service's API (e.g. customer-written software or integrations). - `deprecation`: Advertises the intention to remove service functionality without any change to the operation of the service itself. - `manualTask`: Requires the possibility of manual intervention (running a script, eyeballing configuration, performing database surgery, ...) at the time of upgrade for it to succeed. - `migration`: A fully automatic upgrade migration task with no engineer input required. _Note: only one type should be chosen._

How are new versions calculated?

- ❗The `break` and `manual task` changelog types will result in a major release! - 🐛 The `fix` changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease. - ✨ All others will result in a minor version release.

Type

- [ ] Feature - [ ] Improvement - [ ] Fix - [ ] Break - [ ] Deprecation - [ ] Manual task - [ ] Migration

Description

Add TypeMarker static factory method **Check the box to generate changelog(s)** - [ ] Generate changelog entry

[pkoenig10]

Closing in favor of https://github.com/palantir/conjure-java/pull/2264