CXX-3204 add URI setter for server_selection_try_once

[kevinAlbs]

Summary

• Add method to mongocxx::uri to set serverSelectionTryOnce URI option.

Background & Motivation

Intended to help simplify a a workaround in Atlas Stream Processing. The workaround applies serverSelectionTryOnce=false to a URI if the option is not already specified. I expect this new setter can be used in streams after constructing the mongocxx::uri like:

auto uri = std::make_unique<mongocxx::uri>(uri);
// Set default "serverSelectionTryOnce=false" if not included in URI:
if (!uri->server_selection_try_once()) {
    uri->server_selection_try_once(false);
}

This is the first setter on mongocxx::uri. If acceptable, this pattern can be extended in the future to add setters for more URI options to resolve CXX-2146.

Rejected alternatives

A new option to mongocxx::options::client was considered but rejected. Server selection spec notes:

Multi-threaded drivers MUST NOT provide this mode.

But mongocxx::options::client is applied to both mongocxx::client and mongocxx::pool constructors.

The spec also notes:

Conflicting usages of the URI option and the symbol is an error.

A URI setter avoids possible disagreement.

[vector-of-bool]

LGTM