Add pre-release flag to GitHub releases

[yishaynaPalo]

Description

Currently, chart-releaser doesn't provide a way to mark GitHub releases as pre-releases. This feature would be useful for charts that are still in development or testing phase and shouldn't be considered production-ready.

Proposed Solution

Add a new --pre-release flag to the upload command that allows users to mark GitHub releases as pre-releases.

Implementation Details

The implementation would require:

1. Adding a new boolean flag --pre-release to the upload command
2. Adding a new field PreRelease to the Options struct in the config package
3. Adding a new field PreRelease to the Release struct in the github package
4. Passing the pre-release flag value to the GitHub API when creating releases
5. Updating tests to cover the new functionality
6. Updating documentation in README.md

Benefits

• Allows users to clearly mark pre-release versions of charts
• Provides better integration with GitHub's release features
• Helps users distinguish between stable and unstable chart releases

Acceptance Criteria

• The --pre-release flag is available in the upload command
• When the flag is set, GitHub releases are marked as pre-releases
• Documentation is updated to reflect the new feature
• Tests are added to verify the functionality

[yishaynaPalo]

MR: #580