Support per-field config override in derived implementations

Sometimes the serialization format requires adjusting config only for specific fields. This is easy enough when implementing schemas manually, since config can be adjusted per-call for specific field.

However when using derives it is not available.

As mentioned in https://github.com/anza-xyz/wincode/issues/270 this can be solved by using extra helper schema provider / wrapper and `#[wincode(with = "Helper")]` attribute. However this means that for every possible config adjustment per-field user need to define specialized impl that overrides the config or re-implement the required behavior for given target type.

A more general and flexible approach would be to define config override attribute per field, e.g.
```rust
struct Foo {
a: u64,
#[wincode(config = MyCustomConfig)]
b: u64,
#[wincode(config_with = cfg => cfg.with_varint_encoding())]
c: u64,
}
```

One or both of above attributes would solve the problem, naming TBD.