ci: create GitHub releases for release candidate branches

[tsmithsz]

Problem

The AWS Toolkit VSCode project lacked a streamlined release candidate workflow. Creating release candidates would require manual branch creation and version management.

Solution

Added automated release candidate support:

• Created .github/workflows/setup-release-candidate.yml - Workflow to create RC branches with automatic version bumping

• Supports Major/Minor/Patch/Custom version increments

• Reads and increments both toolkit and amazonq package versions independently

• Creates date-based RC branches (rc-YYYYMMDD)

• Commits version changes and pushes the RC branch

• Updated .github/workflows/release.yml - Added support for release/* branches

• Automatically creates rc-* prefixed releases for release candidate branches

• Proper release naming for RC artifacts

This enables a clean RC workflow: trigger the setup workflow with a commit ID and version increment type, then the release pipeline automatically handles the rest.

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• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[amazon-inspector-ohio]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[amazon-inspector-ohio]

✅ I finished the code review, and didn't find any security or code quality issues.