fix: relax schema_container_of target requirement with ?Sized to allow slices

[dj8yfo]

this blocks

#[near_bindgen]
impl Contract {
    pub fn foo(
        &self,
        #[serializer(borsh)]
        c: &[bool],
    ) {}
}

it has been tested in cargo-near#109 with

[patch.crates-io]
borsh = { git = 'https://github.com/near/borsh-rs.git', branch = "slice_schema_container" }

that this patch works