feat (ai/core): custom request headers (#2116)

Author: lgrammel

.changeset/brown-flowers-smell.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/provider': patch
+---
+
+feat (provider): add headers support to language and embedding model spec

.changeset/fast-peas-applaud.md

@@ -0,0 +1,13 @@
+---
+'@ai-sdk/amazon-bedrock': patch
+'@ai-sdk/google-vertex': patch
+'@ai-sdk/anthropic': patch
+'@ai-sdk/mistral': patch
+'@ai-sdk/cohere': patch
+'@ai-sdk/google': patch
+'@ai-sdk/openai': patch
+'@ai-sdk/azure': patch
+'ai': patch
+---
+
+feat (ai/core): add custom request header support