Allow client certificates for mTLS connections

[plankton6]

Hi, our infrastructure configures the vault server to require mTLS connections, which requires configuring client certificates for the http connection.

Also, our client certificates are encrypted with a passphrase, so they need to be decrypted before configuring them in the reqwest builder.

We started a contribution to this project (now closed), but soon we realised that it was not enough and the implementation was a bit trickier than expected, so we decided to use a custom Client in our side until this feature is fully supported here.

We are still aiming at helping with that, but it definitively requires some guidance from the maintainers:

• How can we decrypt the certificates when the rustls feature is enabled?
• In case that we leave the decryption out of this lib, is it ok to change String to be a Vec<u8> for the settings involving the client cert, key and passphrase? That would allow the client decrypt them and pass the raw bytes to the lib.
• What about changing the ca_cert into a Vec<u8> in case we leave the reading of the CA stack to the user too?

Thanks!