Introduce a Literal package and provide clean utility methods for construction of literals

[jacques-n]

Literals right now are a bit difficult to work with. Constructing them generally requires a bunch of sleuthing. There aren't a lot of docs and protobuf objects are leaked through the provided apis (e.g. ProtoLiteral).

Simple example. Assume we have a variable days which is an int32 of days since epoch.

Current

func NewLiteral[T allLiteralTypes](val T, nullable bool) (Literal, error)

// example usage.
expr.NewLiteral[types.Date](types.Date(days), true)

New Utility Method

func NewDate(days int) (Literal, error)

// example usage.
literal.NewDate(days)

We can also add additional methods for common conversion cases. For example, maybe we add:

NewDecimalFromTwosComplement(scale int, []byte twoscomplement)
NewDecimalFromString(str string)

etc.

As part of this we should also ensure we have reasonable test coverage for the new methods

[jacques-n]

Completed in #41