Add option to allow additionalProperties on generated schemas

## Problem

`jsonschema.ForType` sets `additionalProperties: false` on all object types. There is no way to opt out of this behavior via `ForOptions`.

This is a problem for protocols that send additional fields alongside typed arguments. For example, the [Model Context Protocol](https://github.com/modelcontextprotocol/go-sdk) uses `jsonschema.ForType` to generate tool input schemas. When a protocol envelope includes fields not defined in the tool's Go struct, the strict `additionalProperties: false` causes validation to reject the entire request.

## Use case

The [Ad Context Protocol (AdCP)](https://adcontextprotocol.org) sends protocol-level fields like `adcp_major_version` alongside tool arguments. These fields are not part of any individual tool's schema but are part of the protocol envelope. The strict `additionalProperties: false` rejects these requests before they reach the handler.

The current workaround is to walk the generated schema tree and set `AdditionalProperties = nil` on all object types after generation, which is fragile.

## Proposed solution

Add an option to `ForOptions` that controls whether `additionalProperties: false` is set on generated schemas:

```go
type ForOptions struct {
// ...existing fields...
AllowAdditionalProperties bool
}
```

When `AllowAdditionalProperties` is true, the generated schema would omit `additionalProperties: false`, allowing objects to accept fields not explicitly defined in the Go struct.

## Context

- Filed at the suggestion of @jba in modelcontextprotocol/go-sdk#892
- Current workaround: https://github.com/adcontextprotocol/adcp-go/blob/main/adcp/addtool.go