[LucaCappelletti94]

PostgreSQL lets user-defined types take simple constants or identifiers as modifiers (https://www.postgresql.org/docs/current/sql-createtype.html), and ClickHouse writes Object('json'). DataType::Custom stored each modifier as a bare String, so the quotes were lost on display: Object('json') was printed as Object(json), foo('a b') as foo(a b) (parsed back as two modifiers), and foo('it''s') as foo(it's), which no longer parses.

DataType::Custom now holds a Vec<TypeModifier>, which keeps identifiers (with their quote style), numbers (with the L suffix) and single-quoted strings apart, and strings are displayed with their quotes escaped. This is a public API change for code that builds or matches DataType::Custom or calls parse_optional_type_modifiers. Tests round-trip string, identifier and number modifiers in every dialect, and the ClickHouse Object test now checks the typed modifiers.

This conflicts with #2557, which also builds DataType::Custom modifiers, so whichever lands second needs a small update.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.53%. Comparing base (424378c) to head (cdf700f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2676      +/-   ##
==========================================
+ Coverage   81.52%   81.53%   +0.01%     
==========================================
  Files          42       42              
  Lines       34205    34214       +9     
  Branches    34205    34214       +9     
==========================================
+ Hits        27885    27896      +11     
+ Misses       2770     2768       -2     
  Partials     3550     3550              

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