blob_storage: Make Snapshot type consistent.

[gorzell]

Previously many things treated Snapshot as an opaque type defined by
the request_query macro, but the Blob type was deserializing it as a
DateTime and not reading it from the header at all. Now everything is
using one shared opaque type.

I chose to redefine Snapshot without using the macro so that it could
derive Serialize/Deserialize. This is required because in the cases
Snapshot is part of a serialized response body. It is otherwise compatible with the
previous definition. The alternative would have been to change the macro
to do derive Serialize/Deserialize, but since that would be a change to
core and a number of other types, it felt heavy handed.

[bmc-msft]

Can these use statements be added in the existing azure_core entry above?

[rylev]

request_query already supports adding arbitrary attributes to the type. So the following works already in the main branch:

request_query!(
    /// This type could also be a DateTime but the docs clearly states to treat is as opaque so we do not convert it in any way.
    ///
    /// See: <https://docs.microsoft.com/rest/api/storageservices/get-blob>"]
    #[derive(Deserialize, Serialize)]
    Snapshot,
    "snapshot"
);

[gorzell]

request_query already supports adding arbitrary attributes to the type. So the following works already in the main branch:

Nice, does it still make sense to move it up in the mod tree though?

[rylev]

I think it makes sense to leave it where it was with the rest of the request options, but perhaps I'm thinking about things wrong...

[rylev]

Looks good, but I do have one open question...

[rylev]

What is this needed for? FromStr is most oftened used when something is parsed, but we're not really parsing here - we accept any and all strings...

[gorzell]

This is due to the type signature on get_optional_as:

azure-sdk-for-rust/sdk/core/src/headers/mod.rs

Lines 87 to 93 in 27e1dd4

	pub fn get_optional_as<V, E>(&self, key: &HeaderName) -> crate::Result<Option<V>>
	where
	V: FromStr<Err = E>,
	E: std::error::Error + Send + Sync + 'static,
	{
	self.get_optional_with(key, |s| s.as_str().parse())
	}

which is used here: https://github.com/Azure/azure-sdk-for-rust/pull/1008/files#diff-bdeb8cd7bab3372df4aa2feb49411e6cbaf978c135e2dc43330aff4ee30ed32fR233. There are other functions that could be used like get_optional_str but then you would have to convert that to the snapshot type via into. I am not sure which is better.