Add support for creating and updating repository security advisories

### Describe the feature or problem you'd like to solve

The current `security_advisories` toolset only supports reading advisories (`list_repository_security_advisories`, `list_org_repository_security_advisories`, `get_global_security_advisory`). There is no support for creating, updating, publishing, or requesting CVE IDs for advisories via MCP, forcing security teams to context-switch to the GitHub web UI or REST API for these operations.

### Proposed solution

Add three new tools to the `security_advisories` toolset:

- `create_repository_security_advisory` — create a new draft advisory (`POST /repos/{owner}/{repo}/security-advisories`)
- `update_repository_security_advisory` — update an existing advisory, including transitioning state to `published` (`PATCH /repos/{owner}/{repo}/security-advisories/{ghsa_id}`)
- `request_cve_for_repository_security_advisory` — request a CVE ID from GitHub for a draft advisory (`POST /repos/{owner}/{repo}/security-advisories/{ghsa_id}/cve`)

This would allow security teams to manage the full advisory lifecycle — from draft creation through CVE assignment and publication — entirely within the MCP toolset, without leaving their workflow.

### Example prompts or workflows (for tools/toolsets only)

1. **Create a draft advisory:** "Create a draft security advisory for a stored XSS vulnerability in our Core product affecting versions below 2.1.0, patched in 2.1.0."
2. **Update an existing draft:** "Update advisory GHSA-xxxx-xxxx-xxxx to add the CVSS vector string and change the severity to high."
3. **Request a CVE ID:** "Request a CVE ID from GitHub for advisory GHSA-xxxx-xxxx-xxxx."
4. **Publish an advisory:** "Publish advisory GHSA-xxxx-xxxx-xxxx."
5. **End-to-end workflow:** "Create a draft advisory for an OS command injection vulnerability in our product, request a CVE ID, and publish it once the CVE has been assigned."

### Additional context

**Relevant REST API documentation:**
- [[Create a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#create-a-repository-security-advisory)](https://docs.github.com/en/rest/security-advisories/repository-advisories#create-a-repository-security-advisory)
- [[Update a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#update-a-repository-security-advisory)](https://docs.github.com/en/rest/security-advisories/repository-advisories#update-a-repository-security-advisory)
- [[Request a CVE for a repository security advisory](https://docs.github.com/en/rest/security-advisories/repository-advisories#request-a-cve-for-a-repository-security-advisory)](https://docs.github.com/en/rest/security-advisories/repository-advisories#request-a-cve-for-a-repository-security-advisory)