Add equals operator to Atlas Search

[joykim1005]

Ticket

JAVA-5729

Description

There are several Atlas Search query operators that are not implemented with first class support in the Java driver. This PR adds the equals operator to Atlas Search.

Testing

• ran ./gradlew check
• ran atlas-search-test on Evergreen
• ran SearchOperatorTest
• ran all Evergreen patch

[joykim1005]

com.mongodb.client.ClientSideEncryptionE…estExternal_withExternalKeyVault__false is failing and is unlikely caused by this PR and is happening in others as well.

[katcharov]

Just a couple of comments - possibly no changes needed.

[katcharov]

The docs from our docs page are confusing. I think we should say, ...that searches for documents where a field matches the specified value. in all of these

[stIncMale]

Based on experience, MongoDB official documentation is not always exhaustive or correct. So sometimes, when I suspect that it may be missing something, or that it may not reflect the reality, I check the actual server behavior.

Could we please check whether the equals search operator matches not only the BSON null values, but also unset fields? That is, does the operator

{
    "equals": {
        "path": "job_title",
        "value": null
    }
}

match, for example, the empty document {}, which does not have the field job_title set (or, in other words, does not contain the job_title key). The best way to check this seem to be by introducing an integration test to the PR, as then we will review the code that checks the behavior, and have the answer to the question with us for as long as we have the integration test.

[stIncMale]

If we discover that documents with the field being not set match, then we should rename the equalsNull method into something like equalsNullOrNotSet. I don't have a preference between "not set", "unset", "missing", "absent" (we may want to see if any of these or other words are already used in the Java driver API for expressing the same), but let's not use "undefined" to avoid confusion, as there is a single-value BSON Undefined type.

[katcharov]

If we need to rename it, probably equalsNullOrNotExists (because of the $exists operator)