feat: add new platform api package

[fpdrozd]

This PR adds a new SDK package dedicated to the Spree platform api.
Platform SDK docs.

The monorepo directory structure (packages/) looks like this:

• sdk-core
• sdk-storefront
• sdk-platform
• sdk-fetcher-axios
• sdk-fetcher-node

The npm packages names look like this:

• @spree/core-api-v2-sdk (local)
• @spree/storefront-api-v2-sdk
• @spree/platform-api-v2-sdk
• @spree/axios-fetcher
• @spree/node-fetcher

sdk-core

The platform sdk in huge part uses the same helper functions, error handlers and interfaces as the storefront sdk. In order to avoid code redundancy, all the common parts have been extracted to the sdk-core package. It is configured as a local dependency that doesn't have to be published to npm meaning that storefront and platform sdks include it in their build bundles.

Example

Before:

import squashAndPreparePositionalArguments from '../helpers/squashAndPreparePositionalArguments'
import Http from '../Http'
import type { NoContentResponse, NoContentResult } from '../interfaces/NoContent'
import type { IQuery } from '../interfaces/Query'
import type { IToken } from '../interfaces/Token'
...

With the sdk-core package:

import {
  Http,
  squashAndPreparePositionalArguments
} from '@spree/core-api-v2-sdk'
import type {
  IQuery,
  IToken,
  NoContentResponse,
  NoContentResult
} from '@spree/core-api-v2-sdk'

sdk-platform

Below is a checklist of completed endpoints:

• Addresses
• Adjustments
• Classifications
• Countries
• CMS Pages
• CMS Sections
• Digital Assets
• Digital Links
• Line Items
• Menus
• Menu Items
• Option Types
• Option Values
• Orders
• Payments
• Payment Methods
• Products
• Promotions
• Promotion Actions
• Promotion Categories
• Promotion Rules
• Roles
• Shipments
• Shipping Categories
• Shipping Methods
• States
• Stock Items
• Stock Locations
• Store Credit Categories
• Store Credit Types
• Store Credits
• Tax Categories
• Tax Rates
• Taxons
• Taxonomies
• Users
• Variants
• Webhook Events
• Webhook Subscribers
• Wishlists
• Wished Items
• Zones