Add custom property of traits that have not been registered, yet

In the meeting from 2024-11-12 the idea was presented to support trait definitions that have not been registered, yet. The proposal received some push-back:

- DID method authors who identify an important new trait are inherently incentivized to register this trait in this repository. The definition of a trait with property name, title and description is already 90% of the PR that is required to register the new trait.
- Traits could linger in the custom section forever and create their own "custom" standard.
- Adding additional properties to the traits document is already supported since the defined JSON schema doesn't make additional properties illegal.

Example:

```json
{
"name": "mymethod",
"custom": [
{
"mynewtrait": {
"value": true,
"title": "My new trait",
"description": "This is a super important new trait"
}
}
]
}
```

We will discuss this feature at the upcoming meeting on 2024-11-26.