fix: Fix decoding error when oob attachments contain json data

[conanoc]

Checklist

• I have run swiftlint
• I have run AriesFrameworkTests
• I have run AllTests

Description

Fix #9
Swift Decodable cannot decode untyped json. Requests of oob invitation will be AgentMessage types, but we don't know the exact type in advance.
This fix works as follows to handle this:

• Change the json data of attachments to a String by serializing it
• The string data will be treated the same as the decoded base64-encoded string
• Use JSONSerialization to change the message instead of Codable protocol