Rename `/actuator/httptrace` to `/actuator/httpexchanges` to better
describe its purpose and to remove confusion with distribute tracing.
This change also takes the opportunity to improve the code by making
the `HttpExchange` class (previously `HttpTrace`) fully immutable.
Closes gh-32885
Co-authored-by: Andy Wilkinson <wilkinsona@vmware.com>
3e50836b1a