Unknown validator exception for read_only when using requires/optional with Entity

Hello Grape team,

I am using grape-entity objects for defining my endpoint's parameters, and I ran across this issue.

My entity exposes ID as a read only value, as our back end controls this value

 expose :id, documentation: {type: String, read_only: true}
 # .. other attributes

When defining a PUT request that uses the entity documentation for defining the params it generates an error

Failure/Error: requires :all, using: Entity.documentation.except(:created_at, :updated_at)

Grape::Exceptions::UnknownValidator:
  unknown validator: read_only

I found #2338 which fixed a similar issue for other documentation keys, i.e. is_array, which have been using. I noticed in the PR @mscrivo mentions there maybe other keywords that belong in RESERVED_DOCUMENTATION_KEYWORDS and I wonder if this is one of them?

At first I thought it wasn't, but as a PUT request requires an ID to determine which record to update and our back end controls ID creation, then ID should be marked as read_only to inform users of the API that they cannot change or supply their own value.

If this thinking is correct, then I would be happy to create a PR to include read_only in RESERVED_DOCUMENTATION_KEYWORDS. If not then I will re-evaluate my approach.

Start by tracing Entity.documentation through requires :all and reviewing RESERVED_DOCUMENTATION_KEYWORDS, especially the handling introduced by #2338. Reproduce the unknown-validator error with the issue's Ruby example, then verify the chosen behavior with a regression test; done means read_only is handled as documentation metadata rather than passed to validation.