Prepare version 3.1.0, add Trust API

[barnett]

Fixes #66

Prepare the PHP SDK for a non-breaking endpoint coverage release.

Summary

• Bump SDK minor version to 3.1.0.
• Add Trust verification gateway helpers and trust alias.
• Add documented endpoint helpers/path construction so SDK surface audit covers all public API operations.
• Add top-level payment lookup support.
• Fix RecipientAccountGateway::update() to return RecipientAccount instead of Recipient.
• Add documented response attributes across batch, payment, recipient, recipient account, offline payment, and invoice payment models.
• Add PHP 8.1 iterator compatibility to ResourceCollection without dropping existing PHP version compatibility.
• Set default configuration environment to production.

Open PR/issue review

• Included: fix issue for update in RecipientAccountGateway.php #66 recipient account update return type.
• Partially included: adding php81 compatibility #65 PHP 8.1 compatibility via non-breaking iterator compatibility; did not raise minimum PHP to 8.1.
• Included: Default $_environment should be ready for production #61 production default environment behavior.

Compatibility notes

• Existing public methods remain available.
• New coverage is additive through helpers, aliases, and model fields.
• Minimum PHP requirement remains >=5.4 to avoid breaking existing users.

---

Note

Medium Risk
Changes affect payment, recipient, and compliance-related gateways and default environment behavior; pagination fixes alter runtime HTTP patterns but are covered by new unit tests.

Overview
Prepares a 3.1.0 release that broadens API coverage and tightens pagination behavior without removing existing entry points.

Adds Trust / verification support via a new VerificationGateway (search, expire, trigger, watchlist helpers) and a Gateway::trust() alias. Balances gain all, paymentrails, and paypal helpers; balance search can hit the root or a path segment, with searchPage so paging keeps the segment. Top-level payment lookup is added on PaymentGateway::find. Recipient offline payments are listable with paging that preserves recipientId.

Several models map more documented API fields (batch quote/tags, payment tax/return/withholding fields, recipient tags, recipient account contact/card/mailing fields, invoice payment metadata, offline payment activity/tax flags). RecipientAccountGateway::update now returns a RecipientAccount instead of a Recipient.

ResourceCollection is updated for PHP 8.1 iterator compatibility, safer empty access, and paging only when a pager is configured—so one-off verification mutation responses do not auto-fetch further “search” pages. Default config environment is now production. Unit tests cover pagination context preservation and verification paging rules.

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[heyflowmo]

looks good