feat (client): extensible value type for use with custom parsers

[kevin-dp]

This PR modifies the types of our client to enable users to extend the Value type. This is needed when using custom parsers.

For example, the parser below used to throw a type error because Date is not a Value (where Value is the type of base SQL values):

const parser: Parser = {
  timestampz: (date: string) => {
    return new Date(date)
  },
}

With this PR, we can now write this parser as:

const parser: Parser<Date> = {
  timestampz: (date: string) => {
    return new Date(date)
  },
}

And an example usage of this parser with the ShapeStream:

type CustomRow = {
  foo: number
  bar: boolean
  baz: string
  ts: Date
}

const shapeStream = new ShapeStream<CustomRow>({
  url: `...`,
  parser: {
    timestampz: (date: string) => {
      return new Date(date)
    },
  },
})

Note that this would throw an error as Date is not a type that occurs in the row:

type CustomRow = {
  foo: number
  bar: boolean
  baz: string
}

const shapeStream = new ShapeStream<CustomRow>({
  url: `...`,
  parser: {
    // type error here
    timestampz: (date: string) => {
      return new Date(date)
    },
  },
})

[KyleAMathews]

Ah very nice! I noticed this and it bugged me and now there'll be a fix 💫

[balegas]

Thanks @kevin-dp, that will solve my problem.