export `Float` in index.ts

[john-twigg-ck]

https://github.com/googleapis/nodejs-spanner/blob/main/src/codec.ts#L125

I can convert TO Float using Spanner.float but I cannot easily make an interface to capture ORM being returned from the DB.

Example

// ??????
import { Float } from "@google-cloud/spanner/build/src/codec"
// ??????

export interface Trace {
    traceId: string
    flowName: string

    startTime: ISODateString

    endTime: ISODateString

    // Duration in uSeconds
    duration: Float