chore: Remove unneeded allocations in serializers

[oblique]

This PR reduces the allocations when deserializing in half.

Most of the implemented deserializers was doing a two step deserialization, the first step was to get string value of the field and the second was to convert it to the actual type (i.e. decode base64 to vec). Until now the first step was using String which allocates a new string for the value, however it could just borrow it.

The usual way to do this is <&str>::deserialize(deserializer) whoever, in the case of JSON this will fail if the borrowed string contains the escape character \. Another issue was that Cow::<str>::deserialize always allocates.

A solution was to implement the wrapper CowStr which wraps Cow<str> and uses #[serde(borrow)] which can handle the borrowing of the string.

[zvolin]

LGTM