feat(configwatcher): add write-through methods

[zeevdr]

Summary

• The configwatcher was read-only; applications that wanted to write a config value had to maintain a separate configclient reference.
• Added SetString, SetBool, SetInt, SetFloat methods to Watcher that write to the server via the transport and optimistically update the local Value cache immediately, without waiting for the subscription to deliver the change.
• Added WithDescription and WithExpectedChecksum write options.

Test plan

• go test ./... passes in sdk/configwatcher
• Write a value, verify Value.Get() returns new value immediately
• Write with wrong checksum returns concurrency error from server

Closes #17

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sdk/configwatcher/write.go	85.71%	4 Missing ⚠️

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