Stop replacing Plug Conn params and body_params

[lucacorti]

Hi,

Currently Plug.Conn params and body_params are replaced when casting via OpenApiSpex.Plug.CastAndValidate.
This creates issues with dialyzer (#92) and apparently breaks Plug.Upload (#370) too.

Unfortunately this can't be addressed in a backwards compatible way, so what I'm proposing in this PR is:

• Add casted_body_params and casted_params to conn assigns with the casted version
• Add config flags to disable the old behaviour of modifying Plug.Conn params and body_params.
• Add compilation warnings if the config flag is not set (default) to warn about the breaking change in the next major

This way this change could be immediately merged, and people can start migrating today if they need to.

[lucacorti]

Not sure what is happening on 1.9.

[lucacorti]

Elixir 1.9 is not compatible with OTP 23, so I excluded it from the current test matrix.

https://hexdocs.pm/elixir/1.12/compatibility-and-deprecations.html

[zoten]

Hi everyone! Any chance for this or 398 to enter the main tree?
Are there any blockers? Thanks!

[mbuhot]

@zoten both this PR and #398 use a global application environment flag to toggle the behaviour.
I'd prefer to use a flag passed to the CastAndValidate plug (or perhaps a new plug altogether), allowing controllers to be updated one at a time.

[mbuhot]

Closing in favour of #425