feat: Allow setting log level

[lawmicha]

Issue #, if available:
https://github.com/aws-amplify/aws-appsync-realtime-client-ios/issues/31

Description of changes:
Allow the logging level to be configurable allows the consumers of this package (API Plugin and AppSync SDK) to set it through its log level. In Amplify, when the developer sets Amplify.Logging.logLevel, this can be propagated to AppSyncRealTimeClient.logLevel when the plugins are configured. Since the intention is to propagate the log levels from Amplify, they are identical to the ones defined in Amplify.Logging category https://github.com/aws-amplify/amplify-ios/blob/main/Amplify/Categories/Logging/LogLevel.swift

enum LogLevel: Int {
        case error
        case warn
        case info
        case debug
        case verbose
    }

• errors is the default log level (also is Amplify's)
• the log level can be set with AppSyncRealTimeClient.logLevel = .verbose by developers after calling Amplify.configure if they are debugging and want the logLevels to be different from Amplify.Logging's logLevel, though i don't think this is a very common use case, as developers should just be operating on Amplify.Logging.logLevel

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[royjit]

LGTM