[BE] Using `or` in issue/mr/epic list views and boards

Problem To Solve

When filtering issues, I want to filter by ~"group::portfolio management" OR ~"group::certify", so that I can see issues that have one OR (🤣) the other label.

When filtering issues, I want to filter by assignee is @gweaver OR @hollyreynolds OR @kokeefe, so that I see issues that have...

Proposal

• Add support for an OR attribute in Issuable URL params.
• See this comment for more details

Constraints

• OR is limited to individual types of parameters assignee = me OR you AND label = foo OR bar, not assignee = me OR label = foo

Tier

GitLab Core

Implementation Parts

• IssuableFinder
• API endpoints
• Issue Boards Side

Feature flag

or_issuable_queries

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