Add GraphQL blame endpoint

⚠ Parse span from Gitaly blame result (!128101 - merged) needs to be merged first.

This MR was previously approved but merged incorrectly: !128103 (merged)

What does this MR do and why?

Add a Graphql blame end point.

Expose git blame data via a GraphQL endpoint.
Relies on !128101 (merged) to get blame span value as per this discussion. That MR needs to be merged first

Contributes to #393764 (closed)
Feature flag: #419808 (closed)

How to set up and validate locally

1. Make sure to enable the feature flag graphql_git_blame - Feature.enable(:graphql_git_blame)

2. Create as root create a project called blame-test. Add a file 'test file' and add some text across multiple lines.

3. Perform the following query against either the API; http://127.0.0.1:3000/api/graphql, or the GrapiQL UI; http://127.0.0.1:3000/-/graphql-explorer:

   query TestQuery($fullPath: ID!) {
     project(fullPath: $fullPath) {
       repository {
         blobs(paths: ["test file"]) {
           nodes {
             blame(fromLine: 2, toLine: 6 ){
               firstLine
               groups {
                 lineno
                 lines
                 span
                 commit {
                   sha
                 }
                 commitData {
                   authorAvatar
                   ageMapClass
                   commitLink
                   commitAuthorLink
                   projectBlameLink
                   timeAgoTooltip
                 }
               }
             }
           }
         }
       }
     }
   }

   With query Variables:

   {
     "fullPath": "root/blame-test"
   }

4. You should see the following returned:

MR acceptance checklist

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

Related to #393764 (closed)