Introduce METADATA and WATERMARK syntax

[mwylde]

This PR introduces two new syntax elements to improve our SQL UX, and also makes the meaningless STORED keyword optional for generated fields. The sqlparser side of these changes is in ArroyoSystems/sqlparser-rs#1.

WATERMARK FOR

Allows users to configure the event-time column and watermark generation expression without needing to create and then reference virtual fields:

WATERMARK FOR fieldname [AS watermark_expr]

for example,

CREATE TABLE logs (
  timestamp TIMESTAMP NOT NULL,
  id INT,
  WATERMARK FOR timestamp AS timestamp - INTERVAL '5 seconds'
) WITH (
  ...
)

Previously, this would have been written

CREATE TABLE logs (
  timestamp TIMESTAMP NOT NULL,
  id INT,
  watermark TIMESTAMP GENERATED ALWAYS AS (timestamp - INTERVAL '5 seconds') STORED
) WITH (
  event_time_field = timestamp,
  watermark_field = watermark
)

METADATA FROM

Allow users to configure a column to have metadata injected from the source:

field_name TYPE METADATA FROM 'key'

example:

CREATE TABLE logs (
    id TEXT,
    kafka_topic TEXT METADATA FROM 'topic',
    log TEXT
)

Previous, this would have been written using a special function

CREATE TABLE logs (
    id TEXT,
    kafka_topic TEXT GENERATED ALWAYS AS (metadata('topic')) STORED,
    log TEXT
)

Breaking changes

Existing syntax for both features is still supported, but are deprecated and will be removed in 0.15. However, as WATERMARK is now a keyword, unquoted fields named watermark will now produce an error.