MTC worker: validate bootstrap certs (#113)

Add chain validation for bootstrap certificates, and refactor common chain validation between CT and MTC into the validate_chain_lax function in x509_util.

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Co-authored-by: Michael Rosenberg <mrosenberg@cloudflare.com>

Author: rozbb

Cargo.lock

@@ -239,4 +239,4 @@
             ]
         }
     }
-}
+}