feat: add ResettableTimer for simplified timer management

[Marpond]

What changed?
Introduces an abstraction of resetting timers via ResettableTimer in x/resettabletimer.

Why?
Simplifies common timer patterns where timers need to be updated or extended. Previously, we had to manually cancel the existing timer context, handle CanceledError, and create a new timer. With ResettableTimer, a single Reset() call handles this automatically.

How did you test it?
Added workflow unit tests to ensure functional correctness and deterministic replay behavior.

Potential risks
Low risk. Does not alter current timer behavior, only adds a user-friendly interface built on existing timer primitives.

[jakobht]

Looks good. Added a few suggestions.

I think lets put this in the x/resettabletimer package since it's on top of the basic client

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.80%. Comparing base (3d40460) to head (3e38fdd).

❌ Your project check has failed because the head coverage (83.80%) is below the target coverage (85.00%). You can increase the head coverage or adjust the target coverage.

Files with missing lines	Coverage Δ
x/resettabletimer/resettable_timer.go	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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[jakobht]

I like this example! Nice!