feat(code_suggestions): Added LSP based streaming

Description

Added the ability to stream code suggestions from the LSP.

Related Issues

This is the initial draft for &11722

How has this been tested?

Smoke testing:

1. Start Run Extension VSCode task in this project's VSCode run/debug tab.
2. Enable feature flag: Set "gitlab.featureFlags.streamCodeGenerations": true, in your VSCode user configuration.
3. In a file trigger a generation with a prompt like // Returns an express router that manages DOOM demons

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap