[Config] Add encoder and decoder param to public codable APIs

[ncooke3]

Fix #14368

Relevant: #9084 (comment)

I did not update the following APIs because I wasn't able to get a test case working (See be0616a). I think it's reasonable to limit the scope of exposing the FirebaseData*coders in the public API since it needs some re-thinking).

• firebase-ios-sdk/FirebaseRemoteConfig/Swift/Codable.swift

  Lines 31 to 40 in ea8fb07

  	func decoded<Value: Decodable>(asType: Value.Type = Value.self) throws -> Value {
  	if asType == Date.self {
  	throw RemoteConfigValueCodableError
  	.unsupportedType("Date type is not currently supported for " +
  	" Remote Config Value decoding. Please file a feature request")
  	}
  	return try FirebaseDataDecoder()
  	.decode(Value.self, from: FirebaseRemoteConfigValueDecoderHelper(value: self))
  	}
  	}

• firebase-ios-sdk/FirebaseRemoteConfig/Swift/Value.swift

  Lines 25 to 30 in ea8fb07

  	/// Return a typed RemoteConfigValue for a key.
  	/// - Parameter key: A Remote Config key.
  	/// - Returns: A typed RemoteConfigValue.
  	subscript<T: Decodable>(decodedValue key: String) -> T? {
  	return try? configValue(forKey: key).decoded()
  	}

[paulb777]

LGTM with a short API review document.