Surface the process instance initiator: list column, Views field and filters

Problem/Motivation

Now that a process instance records its initiator (see #3611190), it should be visible and filterable: an operator on the instances list, and a site builder in Views, need to see who started each run and narrow by them. This is also the foundation for a requester 'my instances' surface.

Proposed resolution

• orchestra_interaction_webform: a webform-started run records its submission owner as the initiator; an anonymous submission leaves it unset.
• orchestra_ui: the process-instances list gains an Initiator column (the start-user, linked) and filters by initiator through a user autocomplete.
• orchestra_views: an Initiator field (the uid as a linked username), the raw initiator filtered by the core username autocomplete, and an 'Initiated by the current user' filter for a my-instances list.

The initiator stays a bare uid on the instance (no user entity reference); the field, filter and column resolve the user only for display, so orchestra core keeps no hard dependency on the user module while orchestra_ui and orchestra_views declare it.

Remaining tasks

Review and commit. Follow-up: the requester 'My workflow instances' page and View.