Feature Request: Allow configuration of payload content key for QdrantVectorStore

The Problem
The QdrantVectorStore implementation hardcodes the payload field name for document content as "doc_content". This makes it difficult to integrate Spring AI with pre-existing Qdrant collections that use a different naming convention for the primary text field (e.g., "page_content", "text", etc.).

The key is defined as a private static final String and used in private methods (toDocument, toPayload), preventing simple extension or configuration.

The Desired Solution
Introduce a configuration property to allow users to specify the content field name. This would provide a declarative and straightforward way to align Spring AI with existing data schemas.

A clear example of the desired solution
A user should be able to configure the content key in their application.properties or application.yml file:

The QdrantVectorStore should then use this value when reading from and writing to the Qdrant payload, falling back to the default "doc_content" if the property is not set.

The value of the solution
This feature would significantly improve the out-of-the-box experience for users connecting to existing Qdrant databases, removing the need for boilerplate code (subclassing QdrantVectorStore and overriding methods) just to handle a different field name. It makes the component more flexible and easier to adopt in established environments.

Start with the QdrantVectorStore implementation and trace the private toDocument and toPayload methods where the payload content key is used. Add a configurable property exposed through application.properties or application.yml, preserving "doc_content" as the default, and verify that both reading and writing use the configured field name.