feat: Global additionalProperties validation option

**What version of Ajv you are you using?**

8.20.0

**What problem do you want to solve?**

I want to override how additional (unknown) object properties are handled across *all* schemas validated by an Ajv instance, without editing each schema's `additionalProperties` keyword.

Today the only instance-level control over additional properties is `removeAdditional`, which mutates the data by stripping the extra properties. There is no equivalent option that simply *validates* — i.e. globally rejects additional properties (or globally allows them) while leaving the data untouched.

Concretely:
- I want to enforce a strict "no unknown properties" policy on every object schema (treat `additionalProperties` as `false` everywhere) and get a validation error rather than have the data silently modified.
- Conversely, I sometimes want to relax schemas that declare `additionalProperties: false` and accept the extra properties without changing the schemas themselves.

**What do you think is the correct solution to problem?**

Add a new instance option `additionalProperties` with three values:

- `"default"` (default) — respect each schema's `additionalProperties` keyword (current behavior).
- `"alwaysAllow"` — additional properties are always allowed regardless of the schema; the keyword is treated as `true` (never validated against a subschema, never rejected).
- `"alwaysError"` — additional properties always produce a validation error; the keyword is treated as `false`.

Notes on the semantics:
- Unlike `removeAdditional`, this option never modifies the data — it only affects validation outcomes.
- `"alwaysError"` uses the same trigger model as `removeAdditional: "all"`: it applies to object schemas that contain a `properties`, `patternProperties`, or `additionalProperties` keyword, not to a bare `type: "object"` schema with none of these.
- If `removeAdditional` is also set, it takes precedence (additional properties are removed rather than producing an error).
- The option is ignored for meta-schema validation (added to `META_IGNORE_OPTIONS`), so user schemas legitimately containing extra keywords still compile.

**Will you be able to implement it?**
Yes, PR already open here: https://github.com/ajv-validator/ajv/pull/2637