Manual index annotation for Encodable enums

[elsirion]

As we add and remove enum variants of non-consensus enums over time it will often be easier to accept "holes" in the numbering scheme than writing DB migrations that increment/decrement every variant index by one.

For example:

#[derive(Encodable)]
enum MyEnum {
    Foo, // idx=0
    Bar, // idx=1
    Baz, // idx=2
}

is changed to

#[derive(Encodable)]
enum MyEnum {
    Foo, // idx=0
    Baz, // idx=1
}

now we'd have to write a DB migration that decodes the old Baz values with variant index 2 and re-encodes them with index 1. Instead we could allow the following annotation:

#[derive(Encodable)]
enum MyEnum {
    #[encodable_index = 0]
    Foo, // idx=0
    // note how idx=1 is missing
    #[encodable_index = 2]
    Baz, // idx=2
}