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)

Commencez par l’analyse du parsing des options COPY dans src/parser/mod.rs aux lignes référencées, ainsi que par les définitions de CopyOption et Statement::Copy dans src/ast/mod.rs. Comparez la représentation actuelle avec les exemples d’options de PostgreSQL, DuckDB et DataFusion présents dans l’issue, y compris les clés entre guillemets et les valeurs permissives. Le travail est terminé lorsque les options COPY sont suffisamment génériques pour ces dialectes sans restreindre inutilement les clés ou les valeurs, et que les options héritées restent prises en compte.

Rédigé par le modèle d'indexation à partir du texte de l'issue.