[repository schema] Add repository attribute for application extension ID

[kleihan]

Orchestra has a number of attributes for the <repository> element. One of them (applVerId) is for the application version (not to be confused with the repository version attribute version). It is proposed to add a new attribute applExtId to fully support FIX versioning covered by the FIX fields ApplVerID(1128) and ApplExtID(1156) in the StandardHeader component.

<xs:attribute name="applVerId" type="xs:string"/>
<xs:attribute name="applExtId" type="xs:integer"/>

[kleihan]

This is linked to the existing issue https://github.com/FIXTradingCommunity/fix-orchestra-spec/issues/46 about DC terms to use. That may be a better approach than adding attributes to the root element.