Remove dependency on OpenSSL (#8768)

https://github.com/sigp/lighthouse/issues/8756


  Only the Web3Signer actually needs OpenSSL in order to parse PKCS12 certificates. This updates the function to instead manually parse the cert (using the `p12-keystore` crate) and converts it to a `PEM` certificate (using the `pem` crate) which can be directly converted to a `reqwest::tls::Identity` as this can be done directly in `rustls`.


Co-Authored-By: Mac L <mjladson@pm.me>

validator_client/initialized_validators/Cargo.toml

@@ -12,7 +12,9 @@ eth2_keystore = { workspace = true }
 filesystem = { workspace = true }
 lockfile = { workspace = true }
 metrics = { workspace = true }
+p12-keystore = "0.2"
 parking_lot = { workspace = true }
+pem = "3"
 rand = { workspace = true }
 reqwest = { workspace = true }
 serde = { workspace = true }