COPY TO/FROM is overly restrictive on options

Current parsing of `COPY TO/FROM` options is too restrictive on the how the keys & values can be provided.

https://github.com/sqlparser-rs/sqlparser-rs/blob/ce498864dc705f72e9f85d4dc5f7eba3d17b9ef6/src/parser/mod.rs#L5157-L5180

Given this parsing code, the expected format is:

```sql
COPY (SELECT 1 AS a, 2 AS b) TO 'file'
WITH (
FORMAT 'text',
FREEZE true,
DELIMITER 'd',
NULL 'null',
HEADER true,
QUOTE 'q',
ESCAPE 'e',
FORCE_QUOTE (a, b),
FORCE_NOT_NULL (a, b),
FORCE_NULL (a, b),
ENCODING 'encoding'
)
```

The problems:

- Key must be a keyword and cannot be a quoted string, even though postgres allows this, i.e. `WITH ("format" 'text', ...)`
- It strictly limits the key set at parsing time, leading to further problems:
- Need to update this parser whenever postgres supports a new key, e.g. DEFAULT (supported in postgres 16: https://www.postgresql.org/docs/16/sql-copy.html)
- We can't use this code for other databases/engines with a similar copy statement as they may have different supported keys (e.g. [duckdb](https://duckdb.org/docs/sql/statements/copy.html) and datafusion, see #1080)
- Some values are too strict in their format:
- FORMAT expects an identifier, which disallows escaped strings (`format e'csv'`), but this is allowed by postgres
- FREEZE expects keywords `true` or `false` but postgres allows these to be passed as string literals (single quoted, double quoted, escaped), e.g. `freeze 'true'`
- Same for HEADER

**Solutions**

We could fix these problems for Postgres specifically to make it more permissive, but I was hoping to also fix it to allow usage by other engine dialects (e.g. duckdb and datafusion). This might involve removing the `CopyOption` enum entirely:

https://github.com/sqlparser-rs/sqlparser-rs/blob/ce498864dc705f72e9f85d4dc5f7eba3d17b9ef6/src/ast/mod.rs#L4707-L4736

And replacing its usage in `Statement::Copy` with a more generic version that allows any string key with a new value enum to represent different values (e.g. string value, parenthesized column list value, number value):

https://github.com/sqlparser-rs/sqlparser-rs/blob/ce498864dc705f72e9f85d4dc5f7eba3d17b9ef6/src/ast/mod.rs#L1466-L1478

I'm not sure how the legacy options will fit into this, it could just be left as is and probably tackled as part of another ticket (as the original reason here was for interop with datafusion copy statement)

从参考行中的 src/parser/mod.rs 里的 COPY 选项解析，以及 src/ast/mod.rs 中的 CopyOption 和 Statement::Copy 定义开始。将当前表示与 issue 中 PostgreSQL、DuckDB 和 DataFusion 的选项示例进行比较，包括带引号的键和宽松的值。完成的标准是：COPY 选项对这些方言足够通用，同时不会不必要地限制键或值，并且仍然考虑到 legacy 选项。

由索引模型根据 Issue 内容生成。