feat: implement Dependabot security update tools

[smadi0x86]

Closes: #176

Description

This PR implements tools for managing Dependabot security updates in repositories. The implementation includes:

1. GetSecuritySettings - Retrieves security settings for a repository
2. UpdateSecuritySettings - Updates security settings for a repository
3. GetDependabotSecurityUpdatesStatus - Checks if Dependabot security updates are enabled

Implementation Details

During implementation, I encountered an issue with GitHub's API behavior:

• Public repositories should have Dependabot alerts enabled by default
• However, the API still requires explicit enabling of vulnerability alerts
• This creates a confusing user experience where the system says one thing but behaves differently

As a result, I've:

1. Implemented the core functionality for managing security settings
2. Documented the API behavior discrepancy in the code
3. Commented out the EnableDependabotSecurityUpdates and DisableDependabotSecurityUpdates tools for future investigation.

[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

pkg/github/security.go:131

• Consider adding unit tests to verify that the merge logic in UpdateSecuritySettings handles partial updates correctly.

if securitySettings.AdvancedSecurity != nil {

pkg/github/security.go:182

• Add tests to cover the error handling and successful response cases for GetDependabotSecurityUpdatesStatus.

status, _, err := client.Repositories.GetAutomatedSecurityFixes(ctx, owner, repo)