stricter behavior for parse_json, add try_parse_json, remove to_json

[clintropolis]

Description

This PR adjusts the behavior of PARSE_JSON to be strict and a bit more intuitive, adds a new TRY_PARSE_JSON which will produce NULL values instead of errors for bad or non-string inputs, and removes TO_JSON which no longer had a use since its behavior has been made intrinsic to all JSON expressions.

Previously PARSE_JSON was far far too forgiving, allowing stuff like PARSE_JSON('hello world') to pass through into a string root literal even though it isn't valid JSON. Meanwhile, something like PARSE_JSON('{') would be an error. Basically, the behavior was TO_JSON if the input was not actually JSON, which is far too magical and not particularly useful (apologies this made it in like this, it was a relic of an earlier iteration).

Now, PARSE_JSON is strict and will fail with an error if provided non-string input (with the exception of null, which will output a null) or if the string contains invalid JSON. TRY_PARSE_JSON has also been added, which will return NULL for anything that is not a string containing valid JSON in the cases where PARSE_JSON would have resulted in an error.

I made some adjustments to StructuredDataProcessor to look for and 'unwrap' any StructuredData objects it happens to encounter, which would allow for the JSON expressions which produce COMPLEX<json> outputs to return them as StructuredData, but have not actually made this change to the expressions at this time. This change should be harmless, but isn't currently necessary so I can revert it if any reviewers wish.

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This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.

[vogievetsky]

I have not had the pleasure yet of checking out this branch and playing with it but from the description provided it sounds great. Very excited for TRY_PARSE_JSON!

[cryptoe]

LGTM one minor comment regarding docs.
+1 non binding

[cryptoe]

Is there a doc update required for this ?

[clintropolis]

no, docs in #12922 were written as if this PR were already merged, (to_json_string still exists, but to_json is removed and try_parse_json is added, so this is a result of funny diff stuff i think)

[gianm]

LGTM. The new behavior is more internally consistent, and is therefore an improvement.