Add PPA for dotnet backport during image building

[yhaliaw]

Applicable spec:

Overview

Add the PPA for dotnet backports.

Rationale

Needed to install additional dotnet version for different ubuntu base.
E.g., jammy with dotnet 10, resolute with dotnet 8.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required, documentation)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0
• The application version is incremented in app/pyproject.toml

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[Copilot]

Pull request overview

This PR aims to enable installing additional .NET versions during OpenStack runner image builds by adding the ppa:dotnet/backports repository to the cloud-init provisioning flow. It also includes an unrelated update to the vendored cos_agent charm library affecting scrape config/job naming determinism.

Changes:

• Add ppa:dotnet/backports via add-apt-repository during image build (cloud-init template + unit test expectation).
• Document the change in docs/changelog.md.
• Update vendored lib/charms/grafana_agent/v0/cos_agent.py to generate deterministic scrape config job names and deepcopy generic alert rules (LIBPATCH bump).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
lib/charms/grafana_agent/v0/cos_agent.py	Adds deterministic scrape config job naming + deep-copies alert rule templates; bumps library patch.
docs/changelog.md	Adds a changelog entry for the dotnet backports PPA change.
app/tests/unit/test_openstack_builder.py	Updates expected rendered cloud-init script to include adding the dotnet backports PPA.
app/src/github_runner_image_builder/templates/cloud-init.sh.j2	Adds dotnet backports PPA setup during apt package installation in cloud-init.

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