change Version to struct, prevent misconfiguration of VersioningOverride (#579)

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What changed?
- Make override a one of and add different pinned override types.
- Change version from string to struct
- Make nil Version represent "send tasks away from versioned workers and
to unversioned workers"

Why?
- To prevent misconfiguration and to expand the override definition to
accommodate the new PinnedUntilContinueAsNew behaviors.
- Using a string to represent structured data, whose subfields
frequently need to be considered separately, is brittle and not good for
anyone using it. Struct is better.
- "unversioned version" is contradictory, and nil is a nice way to
represent that the versioning story of a task queue is no longer in the
control of the Deployment whose `RoutingConfig` you are reading.

<!-- Are there any breaking changes on binary or code level? -->
Variables are being deprecated, but the content of the code is not
changing.
The idea of "Unset Ramp" having a separate meaning from "0% Ramp" is
removed. 0% ramp now is the only way to have "no ramp" / 100% of traffic
going to Current Version. Nil `ramping_deployment_version` + non-zero
`ramping_version_percentage` is the way to ramp traffic to unversioned
workers.

The SDK interface for this could look like:
```
deployment := client.GetHandle(deploymentName)

// set current to a version
deployment.SetCurrentVersion(buildId) -> current set to deploymentName/buildId
deployment.UnsetCurrentVersion() -> current set to nil

// set current to unversioned
deployment.UnsetCurrentVersion() -> current set to nil

deployment.SetRampingVersion(buildId, 50) -> ramp set to (deploymentName/buildId, 50)
deployment.UnsetRampingVersion() -> ramp set to (nil, 0%)
deployment.RampToUnversioned(50) -> set to (nil, 50%)
```

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

---------

Co-authored-by: Spencer Judge <[email protected]>
Co-authored-by: Shahab Tajik <[email protected]>

Author: 3 people