Tracking: Enable implemented sqlsmith features

[kwannoel]

Some bugs will cause sqlsmith to fail frequently. So we disable some sqlsmith features temporarily, even though its functionality is implemented.
This issue will track which implemented sqlsmith features can be enabled after its corresponding issue is closed.

Issues:

• Enable NULL: bug(expr): to_char panic with timestamp and null #7327
• Enable Timestamptz: Tracking: Exprs blocked by support of implicit TimeZone of session #5826
• Enable Array and Struct literals: bug: array of struct #6934
• Enable functions in FUNC_BAN_LIST

If some sqlsmith feature should be re-enabled, it will be labelled in the code in the form ENABLE: https://github.com/risingwavelabs/risingwave/issues/XXXX so it is easy to track and enable.

The tracking issue points to the issue for the bug or unsupported feature.

Please do not add issue for requesting new sqlsmith features here. You may create separate issue for those.