Do not use system properties when building the HttpAsyncClient

[jaymode]

This commit removes the usage of system properties for the HttpAsyncClient as this overrides some
defaults that we intentionally change. In order to set the default SSLContext to the system context
we set the SSLContext on the builder explicitly.

Closes #27827

[Tim-Brooks]

I just want to point out that this is slightly different from what is happening now. When it uses the system properties, Apache is calling SSLContexts.createSystemDefault(). Which does:

try {
    return SSLContext.getDefault();
} catch (final NoSuchAlgorithmException ex) {
    return createDefault();
}

And createDefault():

try {
            final SSLContext sslcontext = SSLContext.getInstance(SSLContextBuilder.TLS);
            sslcontext.init(null, null, null);
            return sslcontext;
        } catch (final NoSuchAlgorithmException ex) {
            throw new SSLInitializationException(ex.getMessage(), ex);
        } catch (final KeyManagementException ex) {
            throw new SSLInitializationException(ex.getMessage(), ex);
        }

You know better than I if that change is okay. I just wanted to point it out.

[jaymode]

Thanks for looking into this. I was aware of it and the behavior change was intentional; I think the previous method was too lenient in that it silently ignored a bad parameter and swallowed the exception.

[Tim-Brooks]

LGTM. One comment.