User's private activity about budgeting votes

Is your feature request related to a problem? Please describe.
Decidim provides a user activity timeline at /profiles/nickname/activity.

Here the service shows what the user has done within the service and the user can also see privately what they have done in private spaces.

However, in the acitivity feed there is no information about the user's budgeting votes or "orders" as Decidim calls them internally.

Describe the solution you'd like
We would propose two changes to the activity feed:

1. Add a new "visibility scope" to the activities named "private-only" (current visibilities are "public-only", "admin-only" and "all")
2. Show the "private-only" activity log records in the activity feed that concern the given user when they are looking at the activity feed themselves

In addition to the activity feed changes, the budgeting module should store a "private-only" activity log record when the user casts a budgeting vote (or checks out a Decidim budgeting "order").

Describe alternatives you've considered
N/A

Additional context
We have done a similar customization in the budgeting pipeline module which could be used as the basis for this work but the logic would need to be changed for Decidim core because the logic is specific to the module's records.

The module stores the "private-only" activity log record for a custom record named "vote" which can contain one or multiple Decidim budgeting "orders".

Does this issue could impact on users private data?
This does not expose any extra data publicly about the user.

This only helps the user to find that they voted in PB afterwards if they later login to the platform.

Funded by
We are likely implementing this feature to a client, so we can provide this functionality to the core if agreed.