Refactor VolunteerProject API get to be less custom

[ginkgomzd]

There was a bug in VolunteerProject.getList preventing search-input from being supplied.
...Select lists allowed browsing the list of projects, but search input resulted in no results.

At first, it was determined our custom implentation of api_get was not supporting alternate WHERE comparison operators (other than '=').

This appeared an easy enough enhancement... but then it was revealed that WHERE IN was not getting properly generated SQL. "WHERE ( id IN 1,2,3 )" , i.e. the parens were not provided for the param list.

@eileenmcnaughton says:
ideally you would implement the public function addSelectWhereClause() in your BAO & then basic_get would do the perms

  /**

    Generates acl clauses suitable for adding to WHERE or ON when doing an api.get for this entity *
    Return format is in the form of fieldname => clauses starting with an operator. e.g.:
    @code
    array(
    'location_type_id' => array('IS NOT NULL', 'IN (1,2,3)')
    )
    @endcode *
    Note that all array keys must be actual field names in this entity. Use subqueries to filter on other tables e.g. custom values. *
    @return array */ public function addSelectWhereClause() { ```

more info:
https://github.com/civicrm/civicrm-dev-docs/issues/647