Expose valid enum values as slice in Go SDKs

[nyergler]

Feature Description

We use enumerations in several places in the Polytomic API in order to guide users toward valid values. Fern exposes these as a Go type (great!), but doesn't provide a way to get the full set of valid values. For example, the SyncMode type defined here defines constants for each valid value, but doesn't define a way to get the set of valid values. This would be useful when building other tools to drive the API using the SDK.

Proposed Generated Code (Optional)

Using the SyncMode example, I can imagine one of two approaches:

• simply defining a slice of the valid modes (this would address the base issue)

var ValidSyncModes = []SyncMode{ ... }

• defining a method on the type to get the valid modes

func (m *SyncMode) Values() []SyncMode {
    return []SyncMode{...}
}

The latter feels a tiny bit more useful, since it also allows us to describe the implementation as a generic interface:

type Enum[T any] interface {
    Values() []T
}

(Maybe the method should be ValidValues or EnumValues? I feel less invested in the spelling.)