Allow the ProjectBrowser render element to control which sort options are shown

Problem/Motivation

Similar to #3511069: Allow the ProjectBrowser render element to customize the available filtering options , we want the ProjectBrowser render element to be able to control which sort options are exposed.

Proposed resolution

Introduce a #sort_options property to the render element. If provided, it should be an associative array like the one returned from ProjectBrowserSourceInterface::getSortOptions():

'#sort_options' => [
  'a_z' => $this->t('Alphabetical'),
  'z_a' => $this->t('Reverse alphabetical'),
];

Every key in the array needs to also be defined by getSortOptions() (using array_diff_key()), or we throw an exception. If it passes that bit of validation, then this is provided to the Project Browser instance settings' sorts property. This allows render element calling code to override the names of the sorts, but not their identifiers.

Meanwhile, on the frontend, we don't need to change much; all we have to do is hide the "Sort by" field if there are fewer than 2 sort options.

This should also be configurable for the Project Browser block -- for now, in a simple way, similar to the filters, where we just allow site admins to choose which sorts to show (as a set of checkboxes), but not influence their names or ordering.