Shared Endpoint support in ServerEndpointConfig.Builder

A new method should be added to ServerEndpointConfig.Builder to facilitate easy creation of ServerEndpointConfig instances that use a shared Endpoint instance:

For example:

ServerEndpointConfig config = ServerEndpointConfig.Builder
.create(ProgrammaticEndpoint.class, "/bar")
.subprotocols(subprotocols)
.singleThreaded(false).build();

would create a ServerEndpointConfig instance that used a default Configurator instance that shared a single Endpoint instance between all calls to getEndpointInstance(...)
#### Affected Versions
[1.0]

Start by inspecting ServerEndpointConfig.Builder and its existing Configurator handling. Determine the API shape for creating a configuration that reuses one Endpoint instance across getEndpointInstance(...) calls, then check the surrounding tests and add coverage showing shared behavior and preserving existing builder behavior.

Written by the indexing model from the issue text.