docs: add feature_request.yml issue template

[zeevdr]

Summary

Adds a YAML form-based feature request template to decree, matching the structure and tone of the existing bug_report.yml. Resolves the inconsistency where decree previously had only a bug template, causing feature asks to be filed as bugs or routed to Discussions.

Fields (all match the acceptance criteria in the issue):

• Problem (required)
• Proposed solution (required)
• Alternatives considered
• Component dropdown (Server / CLI / Go SDKs / Proto / Other)
• Additional context

Auto-labels enhancement.

Closes #161

Notes

• The issue references Python/TS feature_request.yml, but those repos currently have feature_request.md (legacy markdown form). This PR uses the richer YAML form to align with decree's existing bug_report.yml. Bringing the SDK repos in line is out of scope here.
• The issue's stretch suggestion (centralize templates in the org .github repo) is left for a follow-up — flagship behavior is more discoverable when the template lives in the repo itself.

Test plan

• YAML parses (python3 -c 'import yaml; yaml.safe_load(...)')
• Verify template renders correctly in GitHub's "New issue" picker after merge