ATLAS-5401: client configuration to support custom token supplier

[mneethiraj]

What changes were proposed in this pull request?

Updated Atlas client for applications to register a customer JWT token provider - using configuration atlas.rest.auth.token.supplier.

How was this patch tested?

• added unit tests for the enhancement introduced in this PR
• verified that all existing tests successfully complete

[Copilot]

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Pull request overview

Adds support for configuring a custom JWT token supplier for Atlas clients via atlas.rest.auth.token.supplier, while keeping the default JWT sourcing behavior working.

Changes:

• Replaces the internal TokenRetriever abstraction with Supplier<String> for JWT retrieval.
• Adds reflective instantiation of a configured token supplier class (with optional Configuration constructor).
• Adds unit tests covering default and custom supplier wiring.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
client/common/src/main/java/org/apache/atlas/token/retriever/TokenRetriever.java	Removes the old token retriever interface in favor of Supplier.
client/common/src/main/java/org/apache/atlas/token/retriever/JwTokenRetrieverDefault.java	Converts default JWT retriever to Supplier<String> and adapts retrieval flow.
client/common/src/main/java/org/apache/atlas/AtlasBaseClient.java	Adds config-driven token supplier loading and uses it for JWT header injection.
client/client-v2/src/test/java/org/apache/atlas/AtlasClientV2Test.java	Adds tests for default/custom token supplier initialization.

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[nixonrodrigues]

Changes looks good. I tested the AtlasClient In IntegrationTest with all variants of JWT token inputs such as -
testBasicAuth

• testJwtFromFile
• testJwtFromEnv
• testCustomTokenSupplier

it looks good.

@Test
    public void testCustomTokenSupplier() throws Exception {
        Configuration configuration = new BaseConfiguration();

        configuration.setProperty("atlas.rest.auth.token.supplier", StaticJwtSupplier.class.getName());

        StaticJwtSupplier.TOKEN = jwtToken;

        AtlasClientV2 client = new AtlasClientV2(configuration, new String[] {ATLAS_URL}, null);

        assertTrue(client.isServerReady(), "isServerReady() should succeed with custom token supplier");
    }

    public static class StaticJwtSupplier implements Supplier<String> {
        static String TOKEN;

        @Override
        public String get() {
            return TOKEN;
        }
    }