Dynamic registration

[ericproulx]

This PR brings a common way to load parsers, formatters, validators and middleware versioners. With this change, it facilitates the customization through inheritance.

Parsers, formatters and error formatters

Parsers, formatters and error formatters had static constant named DEFAULTS that was acting as a lookup to translate a format (:json, :xml, etc ...) to an actual class. Instead of relying on this, it will now use a dynamic registry Grape::Util::Registry that will be warmed up through inheritance like the validators (See base classes) Thus, zeitwerk, eager loads classes, so the registries are filled up when Grape is required.

Versioners

I applied the same pattern for middleware versioners since its translating a symbol to an actual class.

Validators

For the validators, I've just replace the actual registry by Grape::Util::Registry and I removed the const_get part since all validators are eager loaded.

Contract scope

I moved the contract scope validator to the validators folders and rename it to ContractScopeValidator to have a better short name than validator.

Grape::Util::Registry

#2437 introduced a length validator and users that already have one with the same name might not be aware that its been overridden. Giving the situation, the registry will emit a warning if an entry has changed. This is true for all classes that are using the registry.

DryTypeCoercer

I've refactored the part where its was calling const_get to use a static array.

[dblock]

Should we mention that Grape::XYZ::DEFAULTS no longer exists in UPGRADING?

[ericproulx]

Should we mention that Grape::XYZ::DEFAULTS no longer exists in UPGRADING?

Not sure its relevant since its been added 5 month ago. PR
I'm bringing back the registrable but on a new form.