Summarize proposed changes (v2) MR page frontend work

We need to implement a UI for the 'Summarize proposed changes v2'. It should:

1. Be limited to GitLab Ultimate tier
2. Have a control in the MR actions dropdown
3. Open up a drawer on control click and make a request to API
4. Use either a Rest API or GraphQL endpoint (whichever is implemented first, should be easily swappable) to get the generated summary
5. Draw the summary

Availability and Testing

• Add feature specs for new controls.
• Ensure E2E package and test is passing in MRs to ensure no regressions.
• Exploratory testing of feature.