refactor: Modernize SDK to use async await for storage access

[cbaker6]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Synchronous calls can cause networking and storage issues. Though previous versions handle these pretty well, issues can happen such as #37 and #46

Approach

Breaking changes... Remove synchronous storage and networking calls. See updated Playgrounds files for more examples.

Examples:

// Old - synchronous
guard let user = User.current else {
  // Returns if User currently isn't logged in
  return
}
print(user)

// New - async/await
do {
  let user = try await User.current()
  print(user)
} catch {
  // Throws if User currently isn't logged in
}

TODOs before merging

• Add tests
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[cbaker6]

@bcbeta this PR should addresses crashes due to threading as the Keychain and Local storage are now actors and are now thread safe.

The PR will will be released in 5.0.0 later this week.

Note that this is a major breaking change and will require updating code to use.