change Version to struct, prevent misconfiguration of VersioningOverride

[carlydf]

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.

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.

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%)

Server PR

[Sushisource]

Nice! Much safer. Having all these deprecated fields is slightly sad, but, that's life. Let's make sure we track actually removing them after things are GA

None of my comments are blocking.

[Sushisource]

Great, happy with this and 10x more obvious than the previous iteration