feat: tokens endpoint

[alexbarnsley]

Summary

https://app.clickup.com/t/86e00w3wy

I didn't add any special handling for minBalance as it's just an additional query/parameter as far as I understand it 🤔

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[Copilot]

Pull request overview

Adds support for the new tokens endpoints to the ArkEcosystem PHP client, exposing a tokens() accessor on ArkClient and a dedicated Tokens API class, with accompanying request/URL-shape tests.

Changes:

• Introduce ArkEcosystem\Client\API\Tokens with methods for listing tokens, fetching a token, holders, transfers, and whitelist.
• Add ArkClient::tokens() accessor to instantiate the new API.
• Add TokensTest coverage for the new endpoints’ request method + URL paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ArkClient.php	Exposes the new Tokens API via tokens() on the client.
src/API/Tokens.php	Implements request methods for tokens-related endpoints (GET + POST variant).
tests/API/TokensTest.php	Verifies the HTTP method + URL path for tokens endpoints.

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