feat: persist message attributes per topic with JSON editing

[krispetkov]

Several improvements to the tool related to attributes:

• Introduce the ability to cache the attributes between messages. They are stored in the browser's localStorage so you do not have to enter them every time when testing. They are being cached per topic.
• Introduce the possibility to specify the attributes as a json object of key-value pairs
• Introduce buttons to switch between original and json modes
• Add buttons to clear all attributes
• Fix display of long values in the attributes