Mark start and end metaschema flags for protocol as required

[inickles]

This replaces the metaschema constraint tests to ensure start and end flags are set and instead marks them as required in flag definition itself the requirement is expressed in the resulting XML and JSON schemas.

The diffs in the resulting schemas:

oscal_complete_schema.xsd:

3088c3088
<       <xs:attribute name="start" type="NonNegativeIntegerDatatype">
---
>       <xs:attribute name="start" use="required" type="NonNegativeIntegerDatatype">
3098c3098
<       <xs:attribute name="end" type="NonNegativeIntegerDatatype">
---
>       <xs:attribute name="end" use="required" type="NonNegativeIntegerDatatype">

oscal_complete_schema.json:

1889a1890,1892
>     "required" :
>     [ "start",
>      "end" ],

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[iMichaela]

Mandating the start and end to always exist was not the intend here. An author should be allowed to not specify neither start or end of the port range. Only when start is specified, an end must be also specified, and vice versa.