Add jwsbb.HeaderParse (#1422)

Author: lestrrat

Changes

@@ -5,9 +5,13 @@ v3 has many incompatibilities with v2. To see the full list of differences betwe
 v2 and v3, please read the Changes-v3.md file (https://github.com/lestrrat-go/jwx/blob/develop/v3/Changes-v3.md)
 
 v3.0.8 UNRELEASED
-  * [jwe/jwebb] (EXPERIMENTAL) Add low-level functions for JWE operations
+  * [jwe/jwebb] (EXPERIMENTAL) Add low-level functions for JWE operations.
   * [jws/jwsbb] (EXPERIMENTAL/BREAKS COMPATIBILITY) Add io.Reader parameter
-    so your choice of source of randomness can be passed. Defaults to crypto/rand.Reader
+    so your choice of source of randomness can be passed. Defaults to crypto/rand.Reader.
+    Function signatures around jwsbb.Sign() now accept an addition `rr io.Reader`,
+    which can be nil for 99% of use cases.
+  * [jws/jwsbb] Add HeaderParse([]byte), where it is expected that the header
+    is already in its base64 decoded format.
 
 v3.0.7 16 Jun 2025
   * [jws/jwsbb] (EXPERIMENTAL) Add low-level fast access to JWS headers in compact

jws/jwsbb/header.go

@@ -40,6 +40,15 @@ func HeaderParseCompact(buf []byte) Header {
 	if err != nil {
 		return &header{err: err}
 	}
+	return HeaderParse(decoded)
+}
+
+// HeaderParse parses a JWS header from a byte slice containing the decoded JSON.
+// You will need to call HeaderGet* functions to extract the values from the header.
+//
+// Unlike HeaderParseCompact, this function does not perform any base64 decoding.
+// This function is experimental and may change or be removed in the future.
+func HeaderParse(decoded []byte) Header {
 	var p fastjson.Parser
 	v, err := p.ParseBytes(decoded)
 	if err != nil {