Add custom ExchangeId generator support

[yhzdys]

Add ExchangeIdGenerator interface to allow users to customize exchange ID generation in HTTP clients.

This change introduces the ExchangeIdGenerator interface, which enables users to provide custom strategies for generating exchange identifiers. This is particularly useful in distributed systems where custom tracing and correlation mechanisms are required.

[rschmitt]

Just for curiosity, how do you plan to use this?

[yhzdys]

Just for curiosity, how do you plan to use this?

@rschmitt Thank you for the feedback. I understand your curiosity.
In my initial PR changes, I planned to implement custom exchangeId by setting it in advance, like this:

// My custom ExchangeId
final String exchangeId = "myexec-001";

final CloseableHttpAsyncClient client = HttpAsyncClientBuilder.create().build();
final SimpleHttpRequest request = new SimpleHttpRequest(Method.GET, URI.create("http://somehost"));
final HttpClientContext context = HttpClientContext.create();
// Sets my custom ExchangeId
context.setExchangeId(exchangeId);
client.execute(request, context, null);

Of course, although this approach is simple to use, it is not appropriate.

---

With the current changes, I will pass the custom exchangeId through a ThreadLocal, as shown below:
Although there is some performance overhead, it is still acceptable for my use case.

final class MyRequestProducer implements AsyncRequestProducer {

    private final String myExchangeId;

    private final HttpRequest httpRequest;
    private final AsyncEntityProducer entityProducer;

    @Override
    public void sendRequest(final RequestChannel channel, final HttpContext context) throws HttpException, IOException {
        // Sets my custom ExchangeId
        MyExchangeIdGenerator.set(this.myExchangeId);
        channel.sendRequest(this.httpRequest, this.entityProducer, context);
    }
}

final class MyExchangeIdGenerator implements Supplier<String> {

    static final ThreadLocal<String> THREADLOCAL = new ThreadLocal<>();

    static void set(final String exchangeId) {
        THREADLOCAL.set(exchangeId);
    }

    @Override
    public String get() {
        final String myExchangeId = THREADLOCAL.get();
        if (myExchangeId == null) {
            return ExecSupport.getNextExchangeId();
        }
        THREADLOCAL.remove();
        return myExchangeId;
    }
}

final String exchangeId = "myexec-001";
final Supplier<String> exchangeIdGenerator = new MyExchangeIdGenerator();

final CloseableHttpAsyncClient client = HttpAsyncClientBuilder.create().setExchangeIdGenerator(exchangeIdGenerator).build();
final SimpleHttpRequest request = new SimpleHttpRequest(Method.GET, URI.create("http://somehost"));
final MyRequestProducer producer = new MyRequestProducer(exchangeId, request, null);
client.execute(producer, null, null);

[ok2c]

// Sets my custom ExchangeId
context.setExchangeId(exchangeId);
client.execute(request, context, null);

@yhzdys I was actually going to propose that as a simpler alternative, but your current approach is more generic and is fine with me as well

@rschmitt Do you have any objections to merging the PR?