Update additional props handling for custom event for SDK and Iglu

Problem

Currently, the custom event schema accepts any additional properties in addition to the name property. It is hard to differentiate between the name property and any additional properties in Clickhouse, since they are all part of the same object. In addition to this the method signature for trackEvent also does not suggest that both the name of the event and its properties exist on the same level.

Suggestion

Modify the custom_event schema to:

1. Add a second props property next to the name property. This property should be able to receive another object
2. Disable additionalProperties
3. Make the name property required

Modify the JS sdk accordingly to play nice with the new schema.