Create a centralized variable for the header name CamelLangChain4jEmbeddingsVector

While renaming the component (cf. https://issues.apache.org/jira/browse/CAMEL-20611), I had to change the header 'CamelLangChainEmbeddingsVector' to 'CamelLangChain4jEmbeddingsVector' three times: in 'langchain4j-embeddings', 'qdrant', and 'Milvus'.

If we don't extract this string into a common dependency and need to rename it again, we will have to revisit multiple dependencies.

Perhaps it would be better to move it to the 'camel-api' module.