(Calcite PPL) `spath` command dynamic columns support

[ykmr1224]

Support dynamic columns use case with spath PPL command.

Example: following spath command will parse doc attribute and extract all the attributes as columns.

source=idx
| spath input=doc

Each item could be any type since JSON attribute could be string/number/array/null

[penghuo]

source=idx | spath input=doc

@ykmr1224 if we add change requirement of spath as

• input: mandatory. The field to scan for JSON data.
• output: The destination field that the data will be loaded to. Default: value of path.
  • optional, if path exist.
  • mandatory, if path does not exist.
• path: The path of the data to load for the object.
  • optional if output exist.
  • mandatory if output does not exist.

then PPL does not need to support dynamic columns. does it sounds as workaround?