[rust] Fixed nullable byte arrays

[ranger-ross]

Fixes one of the issues reported in #20500.

The issue is types with nullable string of bytes

properties:
  myByteArray:
    type: string
    format: byte
    nullable: true

will generate invalid code

struct Foo {
    #[serde_as(as = "Option<serde_with::base64::Base64>")]
    #[serde(rename = "myByteArray", default, with = "::serde_with::rust::double_option", skip_serializing_if = "Option::is_none")]
    pub my_byte_array: Option<Option<Vec<u8>>>,
}

this does not compile due to serde_as not being compatible with serde_with.

error: Cannot combine `serde_as` with serde's `with`, `deserialize_with`, or `serialize_with`.
  --> src/models/credentials.rs:22:5
   |
22 |     #[serde_as(as = "Option<serde_with::base64::Base64>")]
   |     

This PR removes the with and adds 2 Options if the type is a nullable byte array

#[serde_as(as = "Option<Option<serde_with::base64::Base64>>")]
#[serde(rename = "myByteArray", default, skip_serializing_if = "Option::is_none")]
pub my_byte_array: Option<Option<Vec<u8>>>,

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cc: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro

[jonasbb]

This change will deserialize a null into None. double_option converts a null into Some(None).