Add support for Multi-Resource Refresh Token (MRRT)

[Widcket]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR moves the Credentials Manager from a single credentials model to a multiple credentials one, supporting:

• 1 set of app credentials (the existing functionality)
• N sets of API-specific credentials

To this end, two new public methods were added to the Credentials Manager:

// Plus its combine + async/await wrappers

func apiCredentials(forAudience audience: String,
                    scope: String? = nil,
                    minTTL: Int = 0,
                    parameters: [String: Any] = [:],
                    headers: [String: String] = [:],
                    callback: @escaping (CredentialsManagerResult<APICredentials>) -> Void)

func clear(forAudience audience: String) -> Bool

The Authentication API client's renew(withRefreshToken:scope:) method also gained a new audience argument —that defaults to nil— allowing the exchange of a refresh token for API-specific credentials.

🎯 Testing

A complete test suite is included.

[Widcket]

These are the tests for the async/await wrapper.

[Widcket]

These are the tests for the Combine wrapper. Combine is Apple's framework for functional reactive programming.

[pmathew92]

Any particular reason to remove the default value from forceRenewal parameter ?

[Widcket]

So it's explicit at the call site.

[pmathew92]

LGTM