support for headers param in attest functions

Signed-off-by: Brian DeHamer <bdehamer@github.com>

README.md

@@ -63,6 +63,8 @@ export type AttestOptions = {
   // Sigstore instance to use for signing. Must be one of "public-good" or
   // "github".
   sigstore?: 'public-good' | 'github'
+  // HTTP headers to include in request to attestations API.
+  headers?: {[header: string]: string | number | undefined}
   // Whether to skip writing the attestation to the GH attestations API.
   skipWrite?: boolean
 }
@@ -113,6 +115,8 @@ export type AttestProvenanceOptions = {
   // Sigstore instance to use for signing. Must be one of "public-good" or
   // "github".
   sigstore?: 'public-good' | 'github'
+  // HTTP headers to include in request to attestations API.
+  headers?: {[header: string]: string | number | undefined}
   // Whether to skip writing the attestation to the GH attestations API.
   skipWrite?: boolean
   // Issuer URL responsible for minting the OIDC token from which the