Add definitions.json changes to xrpl package

[JST5000]

High Level Overview of Change

The user interface for defining custom definitions needs more work, so I split up #2127 to focus on the binary-codec, with this one focusing on the interface.

Context of Change

With sidechains and many amendments being worked on, we need a better way to define which types & transactions we're able to serialize/deserialize.

Type of Change

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

Before / After

• Added an optional parameter to high level functions to manually specify the XRPL Definitions to use.
• Added tests for modifying definitions

Test Plan

CI Passes

Future Tasks

• Integrate with server_definitions
• Use the Client as the "global state" so users only have to specify the definitions once per connection.
• Automatically determine definitions when client is connected (defaulting to mainnet values)