Add KumulosCheckinClient to interact with contact tracing checkins API

[cgwyllie]

Description of Changes

Add a client to support contact tracing notifications with Kumulos.

let client = KumulosCheckinClient()

// Check in example
do {
    let checkin = try KumulosCheckin(atLocation: "cuppa-joe-main-st", withContacts: [
        KumulosCheckin.Contact(withLastName: "Bloggs", smsNumber: "+123456789", meta: ["c-19+ve": "no"])
    ])

    client.checkIn(checkin, onComplete: { (outcome) in
        switch (outcome) {
        case let .failure(err):
            print(String(describing:err))
            break
        case let .success(result):
            print(String(describing: result))
            break
        }
    })
} catch {
    print(String(describing:error))
}

// List example
client.getOpenCheckins { (outcome) in
    switch (outcome) {
    case let .failure(err):
        print(String(describing: err))
        break
    case let .success(checkins):
        print(String(describing: checkins))
        break
    }
}

// Check out group example
let checkin = ... // Value from previous list/check in result
client.checkOut(checkins[0]) { (outcome) in
    print(String(describing: outcome))
}

// Check out contact example
let contact = ... // Value from previous list/check in result
client.checkOutContact(checkins[0].contacts[0]) { (outcome) in
    print(String(describing: outcome))
}

Breaking Changes

• None

Release Checklist

Prepare:

• Detail any breaking changes. Breaking changes require a new major version number
• Check pod lib lint passes

Bump versions in:

• Sources/Kumulos.swift
• KumulosSdkSwift.podspec
• KumulosSdkSwiftExtension.podspec
• Build target version for plist
• README.md

Release:

• Squash and merge to master
• Delete branch once merged
• Create tag from master matching chosen version
• Run pod trunk push to publish to CocoaPods