feat(NODE-3083): support aggregate writes on secondaries

[durran]

Description

Allows $merge and $out aggregate operations to execute on secondaries. The logic here is:

1. If the server version is < 5.0, these operations always execute on the primary.
2. If the server version is >= 5.0, these operations operate using the provided read preference or the primary if none is provided.

The spec specifically avoids multi-version clusters, so we can handle this at server selection by implementing our own server selection function for this case that does not need to scan twice.

Includes NODE-3684 to skip $out/$merge tests on serverless

What is changing?

Operations can be flagged with trySecondaryWrite to use the new server selector function. This function looks at the read preference and common wire version of the topology to potentially change the read preference used to select the server.

Is there new documentation needed for these changes?

Potentially if we want to let users know about this new feature in the node docs.

What is the motivation for this change?

DRIVERS-823

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• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests

[dariakp]

Just a question to make sure I understand the implementation (I'll take a second pass at the newly added tests once I better understand the context, but I'll move it to team review)

[nbbeeken]

LGTM

[dariakp]

Just a few small requests regarding the tests; I think we're all happy with the actual code approach, and thank you so much for adding the new unit tests!