pgtype: driver.Valuer support for Array or Map

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**Is your feature request related to a problem? Please describe.**

We use `stdlib` pgx in our project. For unit tests we use [sqlmock](https://github.com/DATA-DOG/go-sqlmock) for unit tests. We want to migrate to `v5` and the related `v5/pgtype` sub-package from the stand-alone `pgtype` package.

sqlmock doesn't like slice types such as `[]int`, `[]string` etc. As a solution we have some wrapper types like:

```
type StringArray []string

// Scan implements the [database/sql.Scanner] interface.
func (s *StringArray) Scan(src any) error {
array := new(pgtype.TextArray)
if err := array.Scan(src); err != nil {
return err
}
if err := array.AssignTo(s); err != nil {
return err
}
return nil
}

// Value implements the [database/sql/driver.Valuer] interface.
func (s StringArray) Value() (driver.Value, error) {
if len(s) == 0 {
return nil, nil
}

array := pgtype.TextArray{}
if err := array.Set(s); err != nil {
return nil, err
}

return array.Value()
}
```

I'm aware that `pgx/stdlib` support direct passing of a slice as parameter value, but `sqlmock` does not. Hence we need to implement the `driver.Valuer` interface like above. The new [`pgtype.Array`](https://pkg.go.dev/github.com/jackc/pgx/v5@v5.4.1/pgtype#Array) does not implement `driver.Valuer` anymore.

I've read the conversation in https://github.com/jackc/pgx/issues/1458. It pointed out, the `Map` type can be used for scanning into arrays, which is fine for that case.

**Describe the solution you'd like**
It would be awesome if one of the following were to be implemented:

1. `pgtype.Array` gets a `Value()` method.
2. `pgtype.Map` gets a `SQLValuer()` or similar method.

**Describe alternatives you've considered**

I'm aware this is not a `pgx` issue necessarily, as we are the ones swapping to another (mock) driver. `sqlmock` provides the possibility to use a [`driver.ValueConverter`](https://pkg.go.dev/github.com/DATA-DOG/go-sqlmock#ValueConverterOption) instead. As some discussions over there suggest, the driver might already export a `ValueConverter` which then can be used. However, I was not able to find anything in the pgx project.

1. If there is a `ValueConverter` somewhere and I missed it, I will gladly use it.
2 Implement my own `ValueConverter` as a last resort.

**Additional context**
none

No source file or test is named. Start by reading the pgtype.Array and pgtype.Map APIs, then compare them with database/sql/driver.Valuer and the stdlib conversion path. Done means a supported way to pass array or map values through sqlmock, with tests covering the relevant parameter conversions.

Written by the indexing model from the issue text.