[Vertex AI] Add internal apiVersion parameter to RequestOptions

[andrewheard]

Added an internal constructor for RequestOptions that supports specifying an API version (e.g., v1 or v1beta).

See go/firebase-vertex-set-api-version (Google-internal only) for more details.

#14405
#no-changelog

[andrewheard]

Removed ExpressibleByStringLiteral conformance for APIVersion since we don't currently have a use-case where using the static properties APIVersion.v1 and APIVersion.v1beta would not be sufficient.

Adding ExpressibleByStringLiteral conformance later, or a static func custom(_ versionIdentifier: String), wouldn't be breaking changes so they've been left out for now.