refactor: create new abstract model for nested dict objects

[mvadari]

High Level Overview of Change

Some objects are of the form:

{
  "Memo": {
    ...
  }
}

xrpl-py abstracts this away and creates the outer dictionary for you. There are a few objects that do this (Memo, Signer, and SignerList). There is a lot of copy-pasted code between these 3 objects. This PR seeks to simplify that by creating a parent object that inherits from BaseModel that handles this extra code.

Context of Change

I have a draft PR that has a few more of these objects, and it avoids having copy-pasted code.

Type of Change

• Refactor (non-breaking change that only restructures code)

Test Plan

CI passes. No change in functionality.

[JST5000]

LGTM

[khancode]

Nice refactor!