Support partially update document by entity.

[puppylpg]

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Closes #2304

[sothawo]

Thanks for this PR. I have commented some minor things, I will fix them with a Polishing commit after merging.

[sothawo]

That change is not necessary for this issue.

[puppylpg]

It's true this modifycation is unrelated with this PR. However,

before this PR, I update an entity using spring-data-elasticsearch in this way:

• to get _id: ElasticsearchRestTemplate#getEntityId
• to get _routing: ElasticsearchRestTemplate#getEntityRouting
• to get _source: elasticsearchConverter.mapObject(entity)

(Then combine id/routing/source to a UpdateQuery.)

The latter two methods have public access while the first one is private thus must be copied from ElasticsearchRestTemplate to my class manually to use. If it was public, this copy won't happen.

If this PR is merged, seems that there is no need to access getEntityId in outer class again. However, getRouting(entity) is similar to this method in the same class and be marked as public, which is convenient to use. I think it may be better to be accessible for this method too.

Anyway, I'll not modify this line to public in new PR, but leave it for your consideration.

[sothawo]

Sorry, had to revert the merge. I missed that it was targeting the wrong branch.
We do not add new features in published versions, only on the main branch, so this should be merged into the main branch.

[puppylpg]

changes are accepted