[Feature][Plugins] Add YouTrack plugin

[tpam28]

Search before asking

• I had searched in the issues and found no similar feature requirement.

Use case

We track work in YouTrack and want its issues in DevLake, so ticket data
(lead time, issue types, status changes) shows up in the same dashboards
as code and CI/CD data.

Description

A new Go plugin youtrack that maps YouTrack data to the TICKET domain layer.

• Connection: YouTrack REST API, permanent token auth.
• Scope: YouTrack projects, mapped to boards.
• Scope config: which custom fields hold Type, State, Priority, Assignee,
  Story Points and Due Date (field names differ between instances), plus
  type and status mappings to the standard values.
• Collects issues, workflow states, comments, changelogs, tags, accounts.
  Incremental collection.
• Config UI: connection form, scope picker, scope config form.
• Grafana dashboards for MySQL and PostgreSQL.
• Unit tests and e2e tests with CSV fixtures.

Related issues

#8468 asked for the same thing. It was closed by the stale bot, and no PR
was ever opened.

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct