base64 parameter engine

[agates]

I'm using poem-openapi and need to accept some base64 public keys for webpush subscriptions -- the format I'm primarily testing with would fit the base64::engine::general_purpose::URL_SAFE_NO_PAD engine but it seems to be defaulting to base64::engine::general_purpose::STANDARD.

#[derive(Debug, poem_openapi::Object)]
struct PushSubscription {
    endpoint: Url,
    p256dh: Base64<Vec<u8>>,
    auth: Base64<Vec<u8>>,
}

Depending on the base64 format I use I get one of the following errors:

parse request payload error: failed to parse "string(bytes)": Invalid padding (occurred while parsing "PushSubscription")
parse request payload error: failed to parse "string(bytes)": Invalid byte 45, offset 45. (occurred while parsing "PushSubscription")

Two questions:

1. Is there a way to specify the base64 engine?
2. Is there a way to allow for multiple engine types?