Spike: Render Blame file information within the Blob viewer frontend app

Context

Recently while discussion an improvement for the Blob viewer (Add line-specific historic information to the s... (#386639)) an idea popped up:

The full blame page could theoretically be achieved in the blob viewer by fetching and displaying blame information for ALL the lines without leaving the page. 🤔 Might be worth opening an issue to discuss how that could be achieved, if we want to dig deeper.

For clarity, the "/blame/" page/url would still exist, it would just render the blob page with the relevant blame information displayed in the gutter.

Proposal

(lacks design guidance)

1. Add a button at the top of the line gutters to "show blame information" (alternatively, this could be the functionality of the BLAME button at the top)
2. That button increases the width of the line gutter and replaces that with the blame information

Motivation

• Maintenance: Doing this in the blob page unifies the code into the Blob vue app
• Performance/UX: Quicker toggling back/forth between blame and normal blob viewing
• Performance/UX:Allows the frontend to selectively request blame data from the backend pertaining to the region currently being shown to the user (and surroundings) resulting in faster performance
• Performance/UX:Reuses information from the blob rendering task so no need to fetch it when toggling the Blame info on.
• Performance/UX:Still allows to render Blame mode from scratch (would essentially render the Blob vue app + Blame on from the start)

Questions

1. backend Do we need new APIs/GraphQL queries? (likely)
2. backend Can we provide blame info in chunks and still do it performantly?
3. frontend Could we be pushing the blob app too far with this? (in terms of resources/performance)
4. frontend Should we tackle this before migrating to Vue 3 or should we wait?
5. UX Does this open up the option to roll out other kinds of interactive feature?

Availability and Testing

Add feature specs for: -opening blame from blob app -ensuring correct blame information is displayed -closing blame from blob app

Exploratory testing of feature.