PostgreSQL, ClickHouse: Keep custom data type modifiers re-parseable with a typed list

[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

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.28%. Comparing base (ada8314) to head (0730482).

Files with missing lines	Patch %	Lines
src/ast/data_type.rs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2676   +/-   ##
=======================================
  Coverage   81.28%   81.28%           
=======================================
  Files          42       42           
  Lines       33920    33929    +9     
  Branches    33920    33929    +9     
=======================================
+ Hits        27571    27579    +8     
  Misses       2785     2785           
- Partials     3564     3565    +1     

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