Accept policies from a directory with multiple yaml files

Currently all policies must we declared in a single file. This can either be a very long error prone file or a script generating the file at runtime. The latter one adds a lot of complexity.

I propose to accept policies from directories like GitLab accepts issue templates and MR templates from .gitlab, triage could accept the following:

1. Read a .triage-policies.yml
2. Read issues rule from: .gitlab/triage/issue/rules
3. Read issues summaries from: .gitlab/triage/issue/summaries
4. Read MR rule from: .gitlab/triage/merge_request/rules
5. ...

Overall structure:

.gitlab/
  triage/
    issues/
      rules/
        Unlabeled.yml
        NoPriority.yml
      summaries/
        NeedsAttention.yml
    merge_requests/
    epics/
    requirements/

This allows big policies sets to be structured in multiple files.

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As an alternative, the yml format might support an import keyword like in gitlab-ci files.