Add GitHub view issue tool

[codegen-sh]

This PR adds a new tool for viewing GitHub issues by their number.

Changes

• Added github_view_issue tool that allows viewing GitHub issues by repository and issue number
• Created necessary directory structure for GitHub tools
• The tool returns detailed issue information including title, body, state, labels, assignees, and comments

Usage

The tool can be used to fetch GitHub issue details:

from codegen.extensions.tools.github.github_view_issue import github_view_issue

result = github_view_issue(codebase, "owner/repo", 123)

This addresses the request to implement a GitHub issue viewing tool.