feat: add feature RPC

[mvadari]

High Level Overview of Change

This PR adds the feature RPC to the models.

Context of Change

There is now a non-admin user version of this RPC: XRPLF/rippled#4781

Type of Change

• New feature (non-breaking change which adds functionality)

Did you update HISTORY.md?

• Yes

Test Plan

Added integration tests.

[ckeshava]

Suggested change

feature?: never

Can we remove this line?
I've seen never used in the context of infeasible control flows. Why do we need to state the absence of a field in an interface?

[mvadari]

It's necessary to distinguish the types. One weirdness about TS types is that you can add on extra fields and it'll still be that type. So this is still a valid FeatureAllRequest object according to TS without this field:

{
  command: 'feature'
  feature: 'something'
}

Adding the never is needed to ensure that this isn't actually a valid FeatureAllRequest type.

The ? is needed because for some reason you get errors for 'feature' field not included in the FeatureAllRequest type if you don't have it.

[mvadari]

cc @justinr1234, our local expert on TS types

[justinr1234]

Will look

[mvadari]

An example in our codebase:

xrpl.js/packages/xrpl/src/models/ledger/BaseLedgerEntry.ts

Lines 18 to 27 in 7e733c4

	export interface MissingPreviousTxnID {
	/**
	* This field is missing on this object but is present on most other returned objects.
	*/
	PreviousTxnID: never
	/**
	* This field is missing on this object but is present on most other returned objects.
	*/
	PreviousTxnLgrSeq: never
	}

[ckeshava]

yes, I did come across other examples in the codebase

[ckeshava]

I only saw control-flow related examples in the Typescript docs website: https://www.typescriptlang.org/docs/handbook/2/functions.html#never

It just feels strange that we are listing non-existent variables in the interface. But I get the rationale as a work-around to typescript's type inference.

This is not a strong objection, but it's nice to not have to do this

[mvadari]

I agree that it's kind of ugly, but I think it's necessary in TypeScript's typing system.