feat(core): use interfaces to assign names to computed types

[sam-goodwin]

We compute types everywhere in Punchcard which can result in some gnarly type signatures that are quite difficult to read.

This happens because we use type aliases instead of using interfaces.

//before
export type ReadOnly<PKey extends keyof A, SKey extends keyof A | undefined, A extends Attributes> = Omit<Client<PKey, SKey, A>, 'put' | 'putBatch' | 'delete' | 'update'>;
// now
export interface ReadOnly<PKey extends keyof A, SKey extends keyof A | undefined, A extends Attributes> extends Omit<Client<PKey, SKey, A>, 'put' | 'putBatch' | 'delete' | 'update'> {}

A type alias has no identifier at compile time. It is simply an alias/pointer to another type. This results in ugly names, especially when using computations like Omit:

Omit<Client<T>, 'putRecord' | 'putRecords' | 'sink'>

Interfaces are the opposite - they give names to things for the TS compiler. This change uses this feature to provide more helpful names to Punchcard types:

// type reported now
Kinesis.Stream.ReadWrite<T>
// before
Omit<Client<T>, 'putRecord' | 'putRecords' | 'sink'>

Breaking Change: Moved the Firehose Validator construct under its own Construct.