Make version optional for versioning override of already-pinned workflows (#687)

_**READ BEFORE MERGING:** All PRs require approval by both Server AND
SDK teams before merging! This is why the number of required approvals
is "2" and not "1"--two reviewers from the same team is NOT sufficient.
If your PR is not approved by someone in BOTH teams, it may be summarily
reverted._

<!-- Describe what has changed in this PR -->
Make version optional for versioning override of already-pinned
workflows


<!-- Tell your future self why have you made these changes -->
So that people don't have to specify a version if they just mean "pin to
what you're already pinned to."

Someone could use this if they want to set pinned overrides on a batch
of workflows that are pinned to different versions, and the user wants
to prevent these workflows from upgrading-on-CaN (since Pinned override
is inherited on CaN, they would not upgrade-on-CaN)


<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**


<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**
temporalio/temporal#8885

Author: carlydf