Bug 33859: Use the phrase 'Identity providers' instead of 'Authentication providers'

Bug 33859: Use the phrase 'Identity providers' instead of 'Authentication providers'

This patch updates instances in the code and templates where the term
"Authentication providers" is used, replacing it with the preferred
"Identity provider."

Most of the instances of this change are in module or API documentation,
but you can see a couple of the changes in the interface:

- Administration -> Identity providers:
  - The sidebar menu should show "Identity providers" instead of
    "Authentication providers."
- Patrons -> Patron details -> More -> Set permissions
  - Under " Manage Koha system settings (Administration panel)" you
    should see "Manage identity providers (manage_identity_providers)"

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>