Require order setting for each realm

[jaymode]

Security has always allowed an order to be specified for realms, but it has never been required. I am wondering if we should consider making this a mandatory setting for each realm so that the user has to explicitly define the order they want and not rely on having a order based on the alphabetical sorting of realm names.

To be specific the behavior I am suggesting would be:

• order is mandatory for each realm definition
• no realms can have the same order value

[elasticmachine]

Pinging @elastic/es-security

[tvernum]

+💯

With type no longer being a field on the realm (it's implied by the key) some realm types no longer have any mandatory settings (in some cases no settings at all) so order has become de facto required for those realms (e.g. native & file) because a realm has to have at least 1 setting.

Making that official for all realms would be so much better.

[tvernum]

A few notes on this:

• This is a breaking change, so it should be 8.0 only.
• We should add an equivalent deprecation in 7.x
• The code changes are likely to be small, but there will be a lot of docs to update.