feat(injector): injector request formation changes (#9306)

Co-authored-by: hyperswitch-bot[bot] <148525504+hyperswitch-bot[bot]@users.noreply.github.com>

Author: su-shivanshmathur

crates/injector/src/consts.rs

@@ -0,0 +1,2 @@
+/// Header name for external vault metadata
+pub const EXTERNAL_VAULT_METADATA_HEADER: &str = "x-external-vault-metadata";