1. If a value is of type ArrayAttribute then append '[]' to the name
   of the value for query parameters (`params`).

This is step 3 in a series of steps of our goal to add full
support for the GitLab API data types[1]:
  * array
  * hash
  * array of hashes

Step one was: commit 5127b1594c00c7364e9af15e42d2e2f2d909449b
Step two was: commit a57334f1930752c70ea15847a39324fa94042460

Fixes: #1698

[1] https://docs.gitlab.com/ee/api/#encoding-api-parameters-of-array-and-hash-types

This can be used to tell if a `RESTObject` was created using
`lazy=True`.

Add a message to the `AttributeError` if attribute access fails for an
instance created with `lazy=True`.

Move the `validate_attrs` function to be inside the `RequiredOptional`
class. It makes sense for it to be part of the class as it is working
on data related to the class.

add exclusive tuple to RequiredOptional data class to support for
mutually exclusive attributes

consolidate _check_missing_create_attrs and _check_missing_update_attrs
from mixins.py into _validate_attrs in utils.py

change _create_attrs in board list manager classes from
required=('label_ld',) to
exclusive=('label_id','asignee_id','milestone_id')

closes https://github.com/python-gitlab/python-gitlab/issues/1897

By having `RequiredOptional` in the `gitlab.base` module it makes it
difficult with circular imports. Move it to the `gitlab.types`
module which has no dependencies on any other gitlab module.

`as_list=False` is confusing as it doesn't explain what is being
returned. Replace it with `iterator=True` which more clearly explains
to the user that an iterator/generator will be returned.

This maintains backward compatibility with `as_list` but does issue a
DeprecationWarning if `as_list` is set.