feat: set tc-php/<version> User-Agent header for Docker API requests

[mdelapenya]

All other testcontainers language libraries (Java, Go, .NET, Node, Python, Rust, Ruby, Elixir) set a tc-<lang>/<version> User-Agent header on Docker Engine HTTP requests. PHP currently sets none.

This PR adds User-Agent: tc-php/<version> to all Docker API requests by:

• Calling DockerClientFactory::createFromEnv() to build the base HTTP client
• Wrapping it in a PluginClient with a HeaderDefaultsPlugin that injects the User-Agent header
• Reading the version at runtime via Composer\InstalledVersions::getPrettyVersion('testcontainers/testcontainers')
• Passing the wrapped client to DockerClient::create()

No new dependencies required — HeaderDefaultsPlugin and PluginClient are already part of the transitive dependency tree via beluga-php/docker-php.

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[rw4lll]

@shyim could you also pls take a look?🙂

[shyim]

Thank you!