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.