Add batch secret retrieval API support

[miewest]

Summary

Adds support for the Conjur Batch Secret Retrieval API (GET /secrets?variable_ids=...) to the Java SDK.

Closes #126

Changes

New: Batch Secret Retrieval

• ResourceProvider: Added retrieveBatchSecrets(String... variableIds) as a default interface method
• ResourceClient: Full implementation — builds the variable_ids query parameter with proper percent-encoding (slashes, @, +, &, spaces as %20), parses the JSON map response via Gson, and strips the {account}:variable: prefix from response keys
• Variables / Conjur: Expose the batch method through the existing delegation chain

Refactored: Endpoints

• Primary constructor is now Endpoints(String applianceUrl, String account) — all service URIs are derived from these two values
• Added Endpoints(String applianceUrl, String account, String authnUrl) for custom authenticators (LDAP, OIDC, etc.)
• Added getBatchSecretsUri() returning {applianceUrl}/secrets
• Factory methods fromSystemProperties() and fromCredentials() simplified to thin wrappers
• Removed redundant getServiceUri() static helper that re-read system properties

Tests

• 26 new unit tests in ResourceClientTest using Mockito to mock the JAX-RS client stack
• Covers: single/multi variable batch, URL encoding edge cases (slashes, @, +, &, spaces), all error codes (401/403/404/422), null/empty args, deeply nested paths, response order preservation, buildBatchQueryParam helper, single secret retrieval, and Endpoints URI derivation

API Usage

Conjur conjur = new Conjur();

// Fetch multiple secrets in one HTTP call
Map<String, String> secrets = conjur.retrieveBatchSecrets(
    "prod/db/password",
    "prod/db/username",
    "prod/api/key"
);

String dbPass = secrets.get("prod/db/password");

Testing

• All 27 unit tests pass (mvn test)
• Manually verified against a live Conjur server (single retrieval, batch retrieval, consistency check, 404 handling)

[miewest]

Ignore close, I mixed up tabs and closed the wrong PR.

[miewest]

I also created a manual full integration test of this code but wasn't sure if it fit in the repo. I can add it if wanted.

[szh]

Thank you for the PR! We've added it to our backlog to review.

Tracking internally as CNJR-13117.

[dondany]

Removing public constructors from Endpoints is technically a breaking change for anyone who instantiated it directly. Since Endpoints is public, consumers had no guardrail telling them not to use it.
Even though Endpoints is not the documented primary
API, it is a public class in a published package — so any external consumer instantiating it directly(for whatever reason) will get a compile error after upgrading.

The old constructors should stay, adjusted for the new functionality.

[dondany]

Same thing as with the other constructor.

[dondany]

This seems like an unnecessary shortcut. This method is already defined in the variables and should be accessed through there.