be consistent with what Nginx supports

[ElvinEfendi]

What this PR does / why we need it:
In https://github.com/kubernetes/ingress-nginx/pull/3409/files we started to apply client_body_buffer_size validator to client_max_body_size directive's value as well. However it turns out client_max_body_size also supports gigabytes using g or G. This introduced and issue #3468 in 0.21.0. In #3492 @aledbf changed the validator to support gigabytes as well, but then it introduces another issue by supporting gigabytes for client_body_buffer_size.

This PR now adds a flag to apply a correct validator for client_body_buffer_size and client_max_body_size instead of applying the same one.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

[ElvinEfendi]

/assign @aledbf

[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, ElvinEfendi

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[cameracker]

Hey guys, thanks for making this change. The removal of this um, semi supported feature did cause an outage for us (not saying this is your fault).

The changelog for the 0.22 release does mention this change, but I would expect this to go under breaking changes because technically this does remove a behavior that was previously available, and requires users to make changes in order to use the new version.

Is there a reason it wasn't included under breaking changes?