feat: Introduce sqlc.optional for dynamic query generation

[copito]

This commit introduces the sqlc.optional feature, allowing conditional inclusion of SQL query fragments at runtime.

Key changes:

1. Parser Enhancement: The SQL parser now recognizes sqlc.optional('ConditionKey', 'SQLFragment') syntax within query files. This information is stored in the query's metadata.

2. Code Generation:

   • Go code generation logic has been updated to process these OptionalBlocks.
   • Generated Go functions now include new parameters (typed as interface{}) corresponding to each ConditionKey.
   • Templates (stdlib/queryCode.tmpl, pgx/queryCode.tmpl) were modified to dynamically build the SQL query string and its arguments at runtime. If an optional Go parameter is non-nil, its associated SQL fragment is included in the final query, and its value is added to the list of database arguments.

3. Parameter Handling: $N placeholders in all SQL fragments (base or optional) consistently refer to the Nth parameter in the generated Go function's signature.

4. Documentation: Added comprehensive documentation for sqlc.optional in docs/reference/query-annotations.md, covering syntax, behavior, parameter numbering, and examples.

5. Examples: A new runnable example has been added to examples/dynamic_query/postgresql/ to demonstrate practical usage.

6. Tests: New end-to-end tests were added in internal/endtoend/testdata/dynamic_query/ for both stdlib and pgx drivers, ensuring the correctness of the generated code.

[ororsatti]

Just a comment, but maybe the optional types can be something like NullString ?

struct xxxOptional {
Xxx *<type>
Valid bool
}

This way it won't have to be interface{} when ever you wanna use a optional value.