Expose struct column extraction helper for AppendStruct inserts

## Observed

1. Existing issue #587 documents that `AppendStruct` fails when a table contains columns that are absent from the Go struct, even when those columns have server-side `DEFAULT` expressions.
2. The current workaround is to explicitly list the inserted columns in `PrepareBatch`, for example `INSERT INTO events (id, name, timestamp)`, so ClickHouse can apply defaults to omitted columns.
3. That workaround is effective, but it duplicates the struct's `ch` tags in raw SQL. In wide or frequently evolving ingestion structs, the column list can drift from the struct definition.

## Expected behaviour

The driver should expose a small helper that extracts the ordered ClickHouse column names represented by a struct, so callers can build explicit insert column lists from the same source of truth used by `AppendStruct`.

This does not need to change `AppendStruct` semantics or add an implicit `DESCRIBE` round trip. It would make the documented workaround for #587 easier to use correctly.

## Code example

```go
package main

import (
"context"
"fmt"
"strings"

"github.com/ClickHouse/clickhouse-go/v2"
)

type Event struct {
ID uint64 `ch:"id"`
Name string `ch:"name"`
}

func insert(ctx context.Context, conn clickhouse.Conn, events []Event) error {
columns, err := clickhouse.StructColumns(Event{})
if err != nil {
return err
}

batch, err := conn.PrepareBatch(ctx, fmt.Sprintf(
"INSERT INTO events (%s)",
strings.Join(columns, ", "),
))
if err != nil {
return err
}
defer batch.Close()

for i := range events {
if err := batch.AppendStruct(&events[i]); err != nil {
return err
}
}
return batch.Send()
}
```

With a table like this, ClickHouse can apply `source` server-side because the generated insert statement only names `id` and `name`:

```sql
CREATE TABLE events (
id UInt64,
name String,
source String DEFAULT 'server default'
) ENGINE = MergeTree()
ORDER BY id;
```

## Error log

Current open-ended insert behaviour still fails as described in #587:

```log
clickhouse [AppendStruct]: missing destination name "source" in *main.Event
```

## Details

### Environment
* [x] `clickhouse-go` version: current `main` and released v2 versions with `AppendStruct`
* [x] Interface: ClickHouse API
* [x] Go version: not version-specific
* [x] Operating system: not OS-specific
* [x] ClickHouse version: any version where the table has `DEFAULT` columns
* [ ] Is it a ClickHouse Cloud?
* [ ] ClickHouse Server non-default settings, if any:
* [x] `CREATE TABLE` statements for tables involved: see code example above
* [ ] Sample data for all these tables, use [clickhouse-obfuscator](https://github.com/ClickHouse/ClickHouse/blob/master/programs/obfuscator/Obfuscator.cpp#L42-L80) if necessary

Related: #587

Start with the existing AppendStruct entry point and the struct tag extraction it already uses, then review related issue #587 for the documented failure mode. Add a public helper that returns the ordered ClickHouse column names represented by a struct, and verify that callers can use it to build explicit insert columns without changing AppendStruct semantics.

Written by the indexing model from the issue text.