JDBC: Support context parameters sent through connection string

[gianm]

In JDBC there are two common ways of setting connection properties, which are used to pass query context parameters through to Druid SQL and Druid native queries. (Additionally, some of them, like "user" and "password", are used for internal purposes by the driver.)

1. Properties object "props" passed into DriverManager.getConnection(url, props).
2. Semicolon-separated key=value pairs appended to the "url" parameter of the same method.

Today (0.18.0) Druid supports (1) but not (2). It would be good to support (2) as well, since while some third-party JDBC applications support both methods of specifying properties, some only support (2).

This is something Avatica should be handling for us, but there is a bug that prevents it from working: https://issues.apache.org/jira/browse/CALCITE-4121.

[gianm]

Hmm, I just realized the Avatica bug is client-side, so updating Druid's version won't help. I think the best way we can address this today is via documentation: #10182 (comment)

Once the Avatica bug is fixed, we can update our docs accordingly.