feat(remote-build): add --project-dir argument to support build paths

[Tejas-Raj01]

Fixes #6287

Describe your changes:

This PR introduces the --project-dir command-line argument for the snapcraft remote-build command to support monorepo setups and custom build paths, fulfilling the requirements outlined in Issue #6287.

Key Technical Details:

• Command-Line Interface: Added the --project-dir option in snapcraft/commands/remote.py, mapping it to the newly supported build_path parameter in upstream craft-application.
• Testing (Unit): Added unit tests in tests/unit/commands/test_remote.py to verify the argument parsing and API calls. A pre-existing test fixture (test_multiple_artifacts_per_build_on) was bypassed for the PLR0917 linter rule using # noqa: PLR0917 to keep the PR scope focused and avoid unrelated refactoring.
• Testing (Spread): Created a new spread test environment under tests/spread/core24/remote-build/ (including task.yaml, arguments.txt, expected-snaps.txt, and dummy project data) to exercise the --project-dir functionality in a remote build scenario.
• Release Notes: Documented the new feature in docs/release-notes/snapcraft-9-0.rst under the "Minor features" section.

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[Tejas-Raj01]

@mr-cal , @medubelko , @cmatsuoka , @bepri
The lint-ruff CI check is failing with 14 PLR0917 (Too many positional arguments) errors. These are occurring in existing test files (test_keys.py, test_lint.py, conftest.py, test_lifecycle.py) that are unrelated to my --project-dir changes.

Since these are mostly pytest fixtures, adding # noqa: PLR0917 to all of them would pollute the PR diff. Would you prefer me to:

1. Add the # noqa tags to these files anyway?
2. Ignore this specific CI failure for this PR?
3. Update pyproject.toml to ignore PLR0917 specifically for the tests/ directory?

Let me know the preferred approach for the repository.