[Java] Extract common timestamp output DoFn

[xujiantop-crypto]

Extract a package-private OutputWithTimestampDoFn shared by
WithTimestamps.AddTimestampsDoFn and
Create.TimestampedValues.ConvertTimestamps.

The existing transform structure is intentionally preserved. In particular,
Create.TimestampedValues still creates an intermediate
PCollection<TimestampedValue<T>>, uses TimestampedValueCoder, restores the
output coder, and retains the ConvertTimestamps transform name. This avoids
the runner matching issue that previously occurred when
TimestampedValues inherited from Create.Values.

WithTimestamps continues to preserve its null timestamp validation and
allowed timestamp skew behavior.

Tests:

• :sdks:java:core:test for WithTimestampsTest and CreateTest
• :runners:direct-java:needsRunnerTest for WithTimestampsTest and CreateTest
• Pipeline renderer tests
• :sdks:java:core:spotlessJavaCheck
• :sdks:java:core:javadoc

Fixes #18350

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.24%. Comparing base (e33cd10) to head (2473d9c).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40077      +/-   ##
============================================
- Coverage     58.52%   57.24%   -1.28%     
+ Complexity     5980     3695    -2285     
============================================
  Files          1459     1200     -259     
  Lines        208448   194031   -14417     
  Branches       5364     3889    -1475     
============================================
- Hits         121985   111070   -10915     
+ Misses        82166    79362    -2804     
+ Partials       4297     3599     -698     

Flag	Coverage Δ
java	72.05% <ø> (-1.15%)	⬇️

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[github-actions]

Assigning reviewers:

R: @kennknowles for label java.

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[kennknowles]

Hello! Thanks for contributing! I'm sorry we left that GitHub issue open. I don't actually think we should do this, for two reasons:

1. We should avoid inheritance with abstract members, and use composition instead.
2. This logic is simple enough it is better to just keep it duplicated in multiple places rather than introduce a dependency between them.