Allow to use non-primary columns in crossref tables

[vol4onok]

Hi guys,

Not sure what was the reason for the change, but in Propel1 it was not mandatory to make the crossref table foreign keys also primary keys. It was handy when the crossref table relation had to be referenced further elsewhere.

I don't see any negative effects enabling this, but please correct me if I'm wrong.

Here is a pseudo-example:

<table name="user_group" isCrossRef="true">
  <column name="id" type="integer" primaryKey="true" required="true" />
  <column name="user_id" type="integer"/>
  <column name="group_id" type="integer"/>
  <foreign-key foreignTable="user">
    <reference local="user_id" foreign="id"/>
  </foreign-key>
  <foreign-key foreignTable="group">
    <reference local="group_id" foreign="id"/>
  </foreign-key>
</table>

<table name="user_group_logs">
    <column name="id"type="INTEGER" primaryKey="true" autoIncrement="true" required="true" />
    <column name="user_group_id" type="INTEGER" />
    <column name="log" type="VARCHAR" />
    <foreign-key foreignTable="user_group">
        <reference local="user_group_id" foreign="id" />
    </foreign-key>
</table>

#1559

[PhilinTv]

@vol4onok thanks for coming back to this long lasting PR!

1. Could you please state what use-cases will be opened with this change?
2. Please provide a relevant test(s) to cover this new use case.

[vol4onok]

@PhilinTv It was taken here #1559. I covered this improvement.
I needed to understand what this fix does In tests everything is clear. This fix didn't work for example in desription.
There are 2 cases. But it's an optional feature and can be released.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.28%. Comparing base (1ca6543) to head (2f29c23).
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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1994   +/-   ##
=========================================
  Coverage     89.28%   89.28%           
  Complexity     8058     8058           
=========================================
  Files           232      232           
  Lines         24534    24534           
=========================================
  Hits          21905    21905           
  Misses         2629     2629           

Flag	Coverage Δ
5-max	89.28% <100.00%> (ø)
7.4	89.28% <100.00%> (ø)
agnostic	68.57% <100.00%> (+1.12%)	⬆️
mysql	69.83% <100.00%> (ø)
pgsql	69.87% <100.00%> (ø)
sqlite	67.81% <100.00%> (ø)

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