Refactor JSON string writing and escaping logic (#638)

This PR significantly improves JSON response serialization (response
projection) performance (50% faster) by replacing the existing
character-by-character string escaping logic with a SIMD-accelerated
implementation adapted from
[sonic-rs](https://github.com/cloudwego/sonic-rs).

I also improved `estimate_final_response_size()` with 20% buffer for
JSON overhead, that according to the tests I did and responses in the
federation audit, is a good estimate.

This code is complex and challenging to maintain, due to the low-level
SIMD operations, unsafe pointer manipulation, and platform-specific
optimizations. However, the previous implementation was also difficult
to maintain in its own way.

The key advantage here is that we're leveraging battle-tested code from
`sonic-rs`, a library we already depend on indirectly.

Author: kamilkisiela

.changeset/improves_json_response_serialization.md

@@ -0,0 +1,8 @@
+---
+executor: patch
+router: patch
+---
+
+# Improve JSON response serialization
+
+This PR significantly improves JSON response serialization (response projection) performance (50% faster) by replacing the existing character-by-character string escaping logic with a SIMD-accelerated implementation adapted from [sonic-rs](https://github.com/cloudwego/sonic-rs).

Cargo.lock

@@ -50,6 +50,7 @@ itoa = "1.0.15"
 ryu = "1.0.20"
 indexmap = "2.10.0"
 bumpalo = "3.19.0"
+sonic-simd = "0.1.2"
 
 [dev-dependencies]
 subgraphs = { path = "../../bench/subgraphs" }