Update to async await

[Andrewangeta]

This PR has a few goals for updating to async await APIs and making certain actions easier to do.
A few key areas of focus:

Async Await

Converting all APIs to use async await which means bumping the swift tools version to 5.7

// Before:
stripe.refunds.create(charge: "ch_12345", reason: .requestedByCustomer).map { ... } 
// After: 
try await stripe.refunds.create(charge: "ch_12345", reason: .requestedByCustomer)

Modifying headers per request

Before:

stripe = StripeClient(...)
stripe.refunds.headers.add(name: "Stripe-Account", value: "acc_12345")
stripe.refunds.create(charge: "ch_12345", reason: .requestedByCustomer)

After:

stripe = StripeClient(...)
stripe.refunds
.addHeaders(["Stripe-Account": "acc_12345",
             "Authorization": "Bearer different_api_key",
             "Stripe-Version": "older-api-version"])
.create(charge: "ch_12345", reason: .requestedByCustomer)

Cleaner type names and public initializers

Remove the Stripe prefix from resources/models as well as make their inits public.
This helps a lot when unit testing to create Stripe types without having to setup a dictionary to use the decoding initializer:

// Before:
let connectProperties = ["id": "acct_1234",
                         "object": "account",
                         "created": Int(Date().timeIntervalSince1970)].toData()
    
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .secondsSince1970
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    return try decoder.decode(StripeConnectAccount.self, from: connectProperties)
    
// After: 
return ConnectAccount(id: "acct_1234", object: "account", created: Int(Date().timeIntervalSince1970))   

What's Been converted

Core Resources

• Balance
• Balance Transactions
• Charges
• Customers
• Disputes
• Events
• Files
• File Links
• Mandates
• PaymentIntents
• SetupIntents
• SetupAttempts
• Payouts
• Refunds
• Tokens

---

Payment Methods

• Payment Methods
• Bank Accounts
• Cards
• Sources

---

Products

• Products
• Prices
• Coupons
• Promotion Codes
• Discounts
• Tax Codes
• Tax Rates
• Shipping Rates

---

Checkout

• Sessions

---

Billing

• Credit Notes
• Customer Balance Transactions
• Customer Portal
• Customer Tax IDs
• Invoices
• Invoice Items
• Plans
• Quotes
• Quote Line Items
• Subscriptions
• Subscription items
• Subscription Schedule
• Test Clocks
• Usage Records

---

Connect

• Account
• Account Links
• Account Sessions
• Application Fees
• Application Fee Refunds
• Capabilities
• Country Specs
• External Accounts
• Persons
• Top-ups
• Transfers
• Transfer Reversals
• Secret Manager

---

Fraud

• Early Fraud Warnings
• Reviews
• Value Lists
• Value List Items

---

Issuing

• Authorizations
• Cardholders
• Cards
• Disputes
• Funding Instructions
• Transactions

---

Terminal

• Connection Tokens
• Locations
• Readers
• Hardware Orders
• Hardware Products
• Hardware SKUs
• Hardware Shipping Methods
• Configurations

---

Sigma

• Scheduled Queries

---

Reporting

• Report Runs
• Report Types

---

Identity

• VerificationSessions
• VerificationReports

---

Webhooks

• Webhook Endpoints
• Signature Verification

[gwynne]

Some remarks

[Andrewangeta]

Addresses #34 #153 #157 #163 #146 #150 #168