PIP-105 Per-subscription properties are not persistent to MetadataStore

[eolivelli]

Describe the bug
When you are using subscription properties, if you restart the broker or you unload the topic, you lose the subscription properties, as they are not stored on the Metadata Service (or in any other place)

To Reproduce

• Create a subscription with properties
• Verify the properties using "pulsar-admin topics stats"
• Restart the broker
• Verify the properties using "pulsar-admin topics stats"
• The properties are lost

Expected behavior
The properties are not lost