Provide a way to deserialize either T or sequence of T

[jonasbb]

From time to time the question comes up how to serialize a string or a list of string. Something like

"key": "value"
// or
"key": ["value0", "value1"]

This can be solved with an untagged enum:

#[derive(serde::Deserialize)]
#[serde(untagged)]
enum OneOrMany<T>{
    One(T),
    Many(Vec<T>),
}

This works but has the "downside" that it remembers if the input was a single value or a list. This distinction is not always necessary. It also hard codes the choice of sequence to a Vec. For this crate the downside is also that the serde_as system doesn't work in such a setup.
The first two can be fixed by writing custom deserialization code which converts both variants into the same collection type.

The idea is to provide a new adapter type which works like this:

#[serde_as(as = "OneOrMany<_>")]
field: Vec<T>,
// complexer setups should also work
#[serde_as(as = "OneOrMany<DisplayFromStr>")]
field: VecDeque<T>,

The OneOrMany will always return a collection type during deserialization and apply the serde_as conversions. For serialization two options are possible: prefer single, or prefer list. In the prefer single case, if the collection has length 1 it serializes as T. In the prefer list case it always chooses to serialize a sequence.

[jonasbb]

The feature is implemented as the OneOrMany type in version 1.7.0.