Reflect DynamicStruct field removal methods

[Diddykonga]

Objective / Solution

Add methods for field removal on DynamicStruct:

DynamicStruct::remove_at( &mut self, index: usize ) -> Option<(Cow<'static, str>, Box<dyn PartialReflect>)>
DynamicStruct::remove_by_name( &mut self, name: &str ) -> Option<(Cow<'static, str>, Box<dyn PartialReflect>)>
DynamicStruct::remove_if( &mut self, f: FnMut((&str, &dyn PartialReflect))->bool ) -> Option<(Cow<'static, str>, Box<dyn PartialReflect>)>

Testing

4 New Tests, located in bevy_reflect/structs.rs:
dynamic_struct_remove_at
dynamic_struct_remove_by_name
dynamic_struct_remove_with
dynamic_struct_remove_combo

---

Showcase

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#[derive(Reflect, Default)]
struct MyStruct {
    a: (),
    b: (),
    c: (),
}

 #[test]
fn dynamic_struct_remove_combo() {
    let mut my_struct = MyStruct::default().to_dynamic_struct();

    assert_eq!(my_struct.field_len(), 3);

    let field_2 = my_struct
        .remove_at(
            my_struct
                .index_of(
                    my_struct
                        .field("b")
                        .expect("Invalid name for `my_struct.field(name)`"),
                )
                .expect("Invalid field for `my_struct.index_of(field)`"),
        )
        .expect("Invalid index for `my_struct.remove_at(index)`");

    assert_eq!(my_struct.field_len(), 2);
    assert_eq!(field_2.0, "b");

    let field_3_name = my_struct
        .name_of(
            my_struct
                    .field_at(1)
                    .expect("Invalid index for `my_struct.field_at(index)`"),
            )
            .expect("Invalid field for `my_struct.name_of(field)`")
            .to_owned();
    let field_3 = my_struct
        .remove_by_name(field_3_name.as_ref())
        .expect("Invalid name for `my_struct.remove_by_name(name)`");

    assert_eq!(my_struct.field_len(), 1);
    assert_eq!(field_3.0, "c");

    let field_1_name = my_struct
        .name_at(0)
        .expect("Invalid name for `my_struct.name_at(name)`")
        .to_owned();
    let field_1 = my_struct
        .remove_if(|(name, _field)| name == field_1_name)
        .expect("No valid name/field found for `my_struct.remove_if(|(name, field)|{})`");

    assert_eq!(my_struct.field_len(), 0);
    assert_eq!(field_1.0, "a");
}

[github-actions]

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[MrGVSV]

A couple nits but also I don't think we should include the other new methods on Struct (at least not part of this PR).

[Diddykonga]

I split this PR between the Dynamic Struct removal methods and the Struct QOL #22708, to be merged after this one.

[alice-i-cecile]

Please update the MSRV in this PR, and remove the lint warning. Once that's done, this LGTM.