Add query element wrappers: Partition, CheckQuery, Assignment, Constraint, TTLElement

[kyleconroy]

This commit adds wrapper types for several AST elements in the EXPLAIN output:

• Partition: Wraps partition expressions in ALTER commands (ATTACH/DROP/DETACH/REPLACE/FREEZE PARTITION)
• CheckQuery: New statement type for CHECK TABLE statements
• Assignment: Wraps column assignments in ALTER UPDATE statements
• Constraint: Wraps constraint expressions in CREATE TABLE
• TTLElement: Wraps TTL expressions in CREATE TABLE

Changes:

• ast/ast.go: Add CheckQuery struct
• parser/parser.go: Add parseCheck() for CHECK TABLE parsing
• internal/explain/statements.go: Add wrapper output for Partition, Assignment, Constraint, TTLElement
• internal/explain/explain.go: Add CheckQuery case in Node dispatch
• Updated metadata.json files to reflect newly passing explain tests