fix: add sync.RWMutex to headerforwarder map

[potterbm-cb]

Motivation

This utility was part of a proof-of-concept and this is an attempt to move it closer to a production-ready state

Solution

Header forwarder was failing with fatal error: concurrent map writes when under heavy load. This change adds a read/write aware locking mechanism to prevent concurrent writes (concurrent reads will not block unless there is a write happening)

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1