Add reviewer input to bump-gitstream-core workflow [skip ci]

[MishaKav]

✨ PR Description

Purpose: Add configurable reviewer option to bump-gitstream-core workflow to support deployment after GitHub SSO implementation.
Main changes:

• Added optional reviewer input parameter to workflow to replace hardcoded github.actor
• Created separate REVIEWER_ARG environment variable for PR creation command
• Modified gh pr create command to conditionally include reviewer based on input parameter

Generated by LinearB AI and added by gitStream.
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[Copilot]

Pull Request Overview

This PR modifies the bump-gitstream-core GitHub workflow to make reviewer assignment configurable. Instead of automatically assigning the workflow actor as a reviewer, users can now specify a custom reviewer via an optional input parameter.

• Adds an optional reviewer input parameter to the workflow
• Conditionally sets reviewer argument based on input parameter
• Replaces hardcoded reviewer assignment with configurable option

[Copilot]

Setting REVIEWER_ARG to empty string in the auto-deploy step will override any reviewer setting. This step should only set LABEL_ARG since REVIEWER_ARG is handled separately in the 'Set reviewer' step.

Suggested change

echo "REVIEWER_ARG=" >> "$GITHUB_ENV"

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[gitstream-cm]

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