announcements: add start at edition date

[gaelgoth]

Hey, I just made a Pull Request!

I reduce a bit the scope of the issue #2416. This PR adds a start_at field, allowing users to set the date when an announcement occurred. For react components, you can choose to sort announcements by either the creation date or the start date, based on your preference.

plugin-announcements-backend

• Add a start_at column. In the migration, backfill the start_at column with the value of created_at for existing announcements
• Introduce new filters: sortBy, which allows fetching announcements sorted by either created_at (default) or start_at. The sort order can be set using the order parameter, with desc (default) or asc.

plugin-announcements

I reworked a bit the New Announcement form to have more room for start at or new other fields in the future:

Announcement start_at info is displayed in small label scheduled in 2 days or occured 2 days ago for past events.

Others changes:

• I added created_at and start_at columns in admin view table
• React props to set which order to display announcements

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-announcements-backend	workspaces/announcements/plugins/announcements-backend	minor	v0.1.4
@backstage-community/plugin-announcements-common	workspaces/announcements/plugins/announcements-common	minor	v0.1.3
@backstage-community/plugin-announcements-react	workspaces/announcements/plugins/announcements-react	minor	v0.1.3
@backstage-community/plugin-announcements	workspaces/announcements/plugins/announcements	minor	v0.1.4

[backstage-goalie]

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