Use @type field to distinguish error detail types

[coryan]

The deserialization for error details assumes that each error detail type can be determined from its structure:

google-cloud-rust/src/gax/src/error/rpc/mod.rs

Lines 282 to 287 in a4cdb98

	#[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
	#[serde(rename_all = "camelCase", untagged)]
	#[non_exhaustive]
	pub enum StatusDetails {
	BadRequest(BadRequest),
	DebugInfo(DebugInfo),

The encoding is an Any:

google-cloud-rust/src/gax/src/error/rpc/mod.rs

Line 45 in a4cdb98

pub details: Vec<StatusDetails>,

That means there should be a @type field with the name of the type. With some custom deserialization we should be able to extract the correct Rust type, without depending on the message structure being different enough.