HTTP 500 raised by POST /shares API when failing to handle "share_type" in request

Description
===========

The POST /shares API [1] does not validate the "share_type" as expected. If the request contains an object instead of a string, HTTP 500 is raised due to a DB lookup error:

Example Request:

        POST https://shared-file-systems.com/share/v2/shares
        Body: {"share": {"share_proto": "cephfs", "description": "share-desc-1579612894", "snapshot_id": null, "name": "share-1663895707", "size": 1, "metadata": {}, "is_public": false, "share_type": {"id": "9b158411-67de-4a90-9b10-92f545d4967f", "name": "type-1726987944", "extra_specs": {"driver_handles_share_servers": "False"}, "required_extra_specs": {"driver_handles_share_servers": false}, "share_type_access:is_public": true, "description": null, "is_default": false}}}

Log from the failure: https://paste.opendev.org/show/809481/

Steps to reproduce
==================

A chronological list of steps which will help reproduce the issue you hit:

* curl -i -X POST $MANILA_ENDPOINT -H "X-Auth-Token: $TOKEN" -H "Accept: application/json" -H "Content-Type: application/json" -d '{"share": {"size": 1, "share_proto": "nfs", "share_type": {"id": "9b158411-67de-4a90-9b10-92f545d4967f", "name": "type-1726987944", "extra_specs": {"driver_handles_share_servers": "False"}, "required_extra_specs": {"driver_handles_share_servers": false}, "share_type_access:is_public": true, "description": null, "is_default": false}}'

Expected result
===============

HTTP 400, BadRequest - the share_type parameter was badly formatted

Actual result
=============

HTTP 500, Internal Server Error

Environment
===========
1. dev branch of openstack/manila

[1] https://docs.openstack.org/api-ref/shared-file-system/?expanded=create-share-detail#create-share