Add acquisition helper methods

[kkostov]

TelemetryDeck/KotlinSDK#76

Alignment with TelemetryDeck/SwiftSDK#230, the following facility methods can be added to the public API:

• acquiredUser( channel: String, parameters: [String: String] = [:], customUserID: String? = nil )

• leadStarted( leadID: String, parameters: [String: String] = [:], customUserID: String? = nil )

• leadConverted( leadID: String, parameters: [String: String] = [:], customUserID: String? = nil )

• onboardingCompleted( parameters: [String: String] = [:], customUserID: String? = nil )

• coreFeatureUsed( featureName: String, parameters: [String: String] = [:], customUserID: String? = nil )

• referralSent( receiversCount: Int = 1, kind: String? = nil, parameters: [String: String] = [:], customUserID: String? = nil )

• userRatingSubmitted( rating: Int, comment: String? = nil, parameters: [String: String] = [:], customUserID: String? = nil )

• paywallShown( reason: String, parameters: [String: String] = [:], customUserID: String? = nil )