[TECH DEBT] Align `Table.key` with Databricks documentation

[JCZuurmond]

Problem statement

Table.key does not align with Databricks documentation, where its called table_reference instead.

Solution

• Rename key attribute to table_reference or sql_table_reference
• Use full_name instead of key when the full name is intended as key/identifier

Originally posted by @nfx in #2955 (comment)