Add sidecar execution, step timeout, and retry support

[vdemeester]

Summary

• Sidecar execution: Sidecars run as Dagger services alongside task steps, accessible by name as hostname
• Step timeout: Steps can specify timeout using Go duration format (30s, 5m, 1h30m)
• Step retry: Steps can specify retries: N for automatic retry on failure

Changes

Sidecar Support

• New pkg/executor/sidecar.go with startSidecars() and bindSidecarsToContainer()
• Sidecars are started before steps and bound via Dagger's WithServiceBinding
• Example: examples/simple/sidecar-pipelinerun.yaml

Timeout Support

• New pkg/executor/timeout.go with parseTimeout() helper
• Added Timeout field to types.Step and parser
• Uses context.WithTimeout for step cancellation
• Example: examples/simple/timeout-pipelinerun.yaml

Retry Support

• New pkg/executor/retry.go with executeWithRetry() helper
• Added Retries field to types.Step and parser
• Logs warning when retries exhausted
• Example: examples/simple/retry-pipelinerun.yaml

Test plan

• All existing tests pass
• New unit tests for sidecar config building
• New unit tests for timeout parsing
• New unit tests for retry logic
• Integration test with sidecar example
• Integration test with timeout example

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