Parse span from Gitaly blame result

What does this MR do and why?

Parse the span information from the Gitaly blame request and adds it and the line number to each blame group.

Span meaning the number of lines within the group which are associated with the blame commit.

This is a requirement for the frontend team discussed here.

Contributes to: #393764 (closed) and is a prerequisite for a GraphQL git blame endpoint: !128103 (merged).

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393764 (closed)