feat: add user setting for code suggestion streaming

Description

Adds a user-facing checkbox for code suggestion streaming.

Should not be merged until these blocking conditions are met: !1268 (comment 1698327995)

Related Issues

issue: gitlab-org/editor-extensions/meta#43 (closed)

How has this been tested?

1. Link to the language server repo, the branch on gitlab-org/editor-extensions/gitlab-lsp!186 (merged) if unmerged, otherwise main
2. Run the LS, observe that the setting exists and can be toggled.
3. Inspect the message sent to the LS, confirm the setting is false by default and true when enabled by the user.

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