Refactor to support all database functions by default

[pseudomuto]

The previous approach required hardcoding function names and argument counts in each driver, making it brittle when databases added new functions or changed arity (e.g., toDateTime64 supporting 1-3 args but only implemented for 1-2). This created maintenance burden and prevented users from accessing newer database functions.

Changes:

• Remove TranslateFunction from Driver interface
• Support any function with any arity at SQL generation time
• Remove 1100+ lines of function translation code from drivers
• Keep Validator for runtime function restrictions when needed
• Remove unused ConcatOperator from Driver interface
• Update documentation to reflect universal function support

Functions are now passed through directly to the database for validation, providing maximum flexibility while maintaining safety through parameterization.

[Copilot]

Pull Request Overview

This PR refactors the function handling architecture to support all database functions by default, eliminating the need for hardcoded function translations in drivers. The previous approach required manually maintaining 1100+ lines of function translation code across drivers, creating maintenance burden when databases added new functions or changed argument counts.

Key changes:

• Remove TranslateFunction requirement from Driver interface
• Build function calls directly in SQL builder without driver involvement
• Remove ConcatOperator from Driver interface as unused
• Update documentation to emphasize universal function support

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sql_builder.go	Implements direct function template building without driver translation
parser.go	Updates function validation comments to clarify parse-time vs runtime validation
functions.go	Updates comments to clarify StandardFunctions is now documentation-only
example_test.go	Updates expected test outputs to reflect new generic function syntax
drivers/postgres/postgres_test.go	Updates test expectations for new function call format
drivers/postgres/postgres.go	Removes 530+ lines of PostgreSQL-specific function translation code
drivers/mysql/mysql.go	Removes 320+ lines of MySQL-specific function translation code
drivers/clickhouse/clickhouse.go	Removes 270+ lines of ClickHouse-specific function translation code
driver_test.go	Removes TranslateFunction and ConcatOperator from mock driver
driver.go	Updates Driver interface to remove function translation methods
README.md	Updates documentation to reflect universal function support

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