feat: add graph-indexed header in subgraph query response

[shiyasmohd]

Fixes #494

[coveralls]

Pull Request Test Coverage Report for Build 12150969087

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• 11 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.04%) to 80.223%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/service/src/routes/request_handler.rs	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
crates/watcher/src/lib.rs	4	92.71%
crates/service/src/routes/status.rs	7	0.0%

Totals
Change from base Build 12090402750:	-0.04%
Covered Lines:	6827
Relevant Lines:	8510

---

💛 - Coveralls

[Theodus]

I literally implemented this just now assuming no one did it yet, and it looks the same. So I'll give this an approval 👍

[shiyasmohd]

@Theodus Thanks for the review. Pushed the changes