Implement Firebird 6.0 schema support

[mrotteveel]

Implement Firebird 6.0 schema support.

This will initially target Jaybird 7, backporting to Jaybird 6 and possibly Jaybird 5 will be considered at a later stage.

[mrotteveel]

Development will initially be done on https://github.com/FirebirdSQL/jaybird/tree/schema-support, which will only be merged after a decision to backport to earlier versions or not.