[FEATURE] Pulsar Order Key

### Why do we need this improvement?

Pulsar message order is an important part to consistently understanding a stream of messages. Pulsar guarantees message order for the receiver using a key or partitioning key, when they use a KeyShared subscription.

See
* https://pulsar.apache.org/docs/next/concepts-messaging/#ordering-guarantee
* https://pulsar.apache.org/docs/next/concepts-messaging/#key_shared

### How will this change help?

Documenting the ordering key clarifies both the ordering guarantees (high-level) and the partitioning (technical). Together, this allows providers and consumers to agree on how messages are partially ordered, leading to consistent understanding on both sides.

### Screenshots

_No response_

### How could it be implemented/designed?

I propose adding either `order-key`, `partition-key` or `key` to the [pulsar bindings](https://github.com/asyncapi/bindings/tree/master/pulsar#message-binding-object).
While my employer defines a consistent ordering on channel level, for multiple operations and message types, Pulsar allows defining any value as ordering key on message level.

As such, similar to [Kafka bindings](https://github.com/asyncapi/bindings/tree/master/kafka#message-binding-object), I propose defining a message binding and adding it there.

### 🚧 Breaking changes

No

### 👀 Have you checked for similar open issues?

- [x] I checked and didn't find a similar issue

### 🏢 Have you read the Contributing Guidelines?

- [x] I have read the [Contributing Guidelines](https://github.com/asyncapi/.github/blob/master/CONTRIBUTING.md)

### Are you willing to work on this issue?

Yes I am willing to submit a PR!

Start with the Pulsar bindings Message Binding Object and compare the Kafka bindings Message Binding Object linked in the issue. Resolve whether the field should be named order-key, partition-key, or key, then add the selected message-level binding and document the ordering and partitioning behavior described by the Pulsar references.

Written by the indexing model from the issue text.