4.x: Unified constants for configuring outbound id and secret.

[tomas-langer]

Description

A single location to store constants that allow overriding of user id and secret (username and password for Basic authentication, user id for JWT).
This PR does not change Digest authentication at all, as we do not support outbound security with digest, and we do not plan to (it is an outdated insecure technology).

Related to #7207

Documentation

We had a few constants in the providers named EP_PROPERTY_OUTBOUND_USER and EP_PROPERTY_OUTBOUND_PASSWORD.
The new location is io.helidon.security.EndpointConfig.
New constants:

• PROPERTY_OUTBOUND_ID - user name, client id to be used for the outbound call
• PROPERTY_OUTBOUND_SECRET - password, client secret to be used for the outbound call

The set of providers supporting these constants is current unchanges (Basic authentication, JWT Authentication provider, and MP JWT Authentication provider).

To use, configure the client request similar to the following:

client.get()
  .uri(uri)
  .property(EndpointConfig.PROPERTY_OUTBOUND_ID, username)
  .property(EndpointConfig.PROPERTY_OUTBOUND_SECRET, password)
  .request();

And if the correct outbound provider is configured with web client security, the provided values will be used.
This also works when security is invoked manually through API (and other clients may support configuraiton of custom properites that are propagated to security).