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

Commit b6447211754e126f64e12fc735ad74fe557b7fb4 inadvertently
introduced a possible breaking change as it added a new argument
`iterator` and added it in between existing (potentially positional) arguments.

This moves the `iterator` argument to the end of the argument list and
requires it to be a keyword-only argument.

* feat(downloads): allow streaming downloads access to response iterator

Allow access to the underlying response iterator when downloading in
streaming mode by specifying `iterator=True`.

Update type annotations to support this change.

* docs(api-docs): add iterator example to artifact download

Document the usage of the `iterator=True` option when downloading
artifacts

* test(packages): add tests for streaming downloads

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.

Replace usage with f-string

There is no reason to return an `int` as we can always return a `str`
version of the `int`

Change `EncodedId` to always return an `EncodedId`. This removes the
need to have `mypy` ignore the error raised.

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