Validate enum types

Overview

Enum types often have specific values they can take. For example, DiskBackendType can only be "local" or "distributed". When receiving external input, it is useful to validate that a value is actually a valid enum variant, so the SDK could provide validation methods like:

func (v DiskBackendType) Valid() bool {
	switch v {
	case DiskBackendTypeLocal, DiskBackendTypeDistributed:
		return true
	}
	return false
}

Now users can just cast an input and have the SDK validate it:

t := oxide.DiskBackendType(input)
if !t.Valid() {
    return fmt.Errorf("invalid disk type")
}

Implementation details

No response

Anything else you would like to add?

No response

Start by locating the enum type definitions in the Go SDK and check whether existing validation patterns or tests cover similar values. Add validation methods for the enum variants described in the issue, then verify that known variants are accepted and unknown cast values are rejected across the affected enums.