Add `src_attr` field in `report_checks`

[akashlevy]

See title. src_attr is the src attribute of an instance in the netlist (useful for tracking an object back to source in pre-synthesis RTL).

Breakdown of work:

• [x] Update Sta to include report_src_attr arg
• [x] Update ReportPath to add src_attr to reporters
• [x] Update Search to add TCL interface to use src_attr field
• [x] Add test to check that src_attr is reported correctly
• [x] Update docs to include src_attr field