[plugin] Move cross-compile backend factory to BuilderConfiguration; fix log handler deprecation warning

[sebsto]

Two small follow-ups on top of the build-backend refactor (#679).

1. Move the cross-compile backend factory onto BuilderConfiguration

The call site read circularly:

backend = try configuration.crossCompileMethod.makeBackend(configuration: configuration)

— reaching through the configuration to get the method, then handing the
configuration straight back. The factory needs three values that the
configuration already owns (crossCompileToolPath, baseDockerImage,
disableDockerImageUpdate) plus the method, so it belongs on the configuration.

• CrossCompileMethod is now purely the parsed user choice (parse + cases +
  description); its makeBackend(...) is gone.
• BuilderConfiguration gains makeCrossCompileBackend() -> any BuildBackend,
  which reads everything it needs from self.
• The call site becomes backend = try configuration.makeCrossCompileBackend().

No behaviour change. Also adopts explicit any (any BuildBackend,
any ContainerCLI).

The backend-selection tests now drive through
BuilderConfiguration(...).makeCrossCompileBackend() (passing --cross-compile).
The old "unsupported methods throw" test on the factory was dropped: unsupported
methods (swift-static-sdk, custom-sdk) are now rejected earlier, at
BuilderConfiguration.init via CrossCompileMethod.parse, and that behaviour is
already covered by UnsupportedCrossCompileMethodsPropertyTests.

2. Fix a deprecation warning in the test log handler

swift-log 1.13 added a log(event:) requirement to LogHandler and deprecated the
old log(level:message:source:...) method. CollectEverythingLogHandler (test
helper) still implemented the deprecated method, so it fell back to the deprecated
default implementation of log(event:), producing a build warning. It now
implements log(event:) directly. (JSONLogHandler in the library already did.)

Verification

• Clean build of products and tests: no compiler warnings originate from our
  sources (Sources/, Tests/); remaining output is only unrelated ld: macOS
  dylib-version notes and the pre-existing Docs.docc "unhandled file" note.
• All tests pass: 140 AWSLambdaRuntimeTests, 83 AWSLambdaPluginHelperTests.
• swift format lint --strict clean.