What this PR does / why we need it:

Changes:

• Add 3 public static factory methods: createWithStub() with
  overloads for timeout and credentials
• Refactor constructors to use delegation pattern with a private
  master constructor that handles both ManagedChannel and stub cases
• Add comprehensive test coverage for new factory methods
• Maintain backward compatibility with existing create() and
  createSecure() methods

Which issue(s) this PR fixes:

Previously, FeastClient could only be instantiated with a
ManagedChannel, requiring users to manage channel lifecycle
directly. This prevented integration with libraries like
eg-spring-boot-clients that abstract channel management and
only expose pre-configured stubs via getClient() methods.

The new factory methods allow users to pass a ServingServiceBlockingStub
directly, enabling seamless integration with such libraries while
maintaining the existing API surface for backward compatibility.

Misc