Fix XDBG tests (partially)

[mkysel]

This fixes upgrade tests and all the plumbing for XDBG.

There are a lot of issues with XDBG that prevent this from being fully operational.

[graphite-app]

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[macroscopeapp]

Fix XDBG tests by adding gateway container support and Docker build script

• Adds new Docker build script dev/docker/build-gateway for creating XMTP gateway service images with git-based versioning
• Introduces XmtpdGatewayContainerBuilder struct in pkg/integration/docker_utils_test.go for configuring gateway containers in integration tests
• Updates XdbgContainerBuilder to support separate gateway and target addresses through new WithGatewayTarget method
• Modifies test setup in pkg/integration/main_test.go to build gateway development images
• Updates XDBG image version from sha-78a5ac2 to sha-26bb960 and removes XMTPD_PAYER_ENABLE environment variable
• Adds gateway container testing to TestLatestVersion in pkg/integration/upgrade_test.go
• Temporarily disables TestXDBGRealMLSPayloads in pkg/integration/xdbg_test.go while adding gateway configuration

📍Where to Start

Start with the new buildGatewayDevImage function in pkg/integration/docker_utils_test.go to understand how the gateway Docker image is built, then examine the XmtpdGatewayContainerBuilder struct in the same file to see how gateway containers are configured for testing.

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Macroscope summarized 161e06c.