Support readPreferenceTags as options array

[MarioCuljak]

This pull requests propose changes to configuration options for MongoClient regarding readPreferenceTags.

Currently, read preference tags can be configured using string of comma-delimited sequence of colon-delimited key/value pairs.
Example:

<?php
$options = [
    'readPreference' => 'secondary',
    'readPreferenceTags' => 'dc:east,use:reporting'
];

$m = new MongoClient("mongodb://localhost/", $options);
?>

There are limitation when using string syntax:

• unable to define multiple tag sets
• unable to define empty tag set

Proposed solution allow readPreferenceTags to be set using array syntax.
Same syntax is used when configuring tags using setter methods.
Example:

<?php

$options = [
    'readPreference' => 'secondary',
    'readPreferenceTags' => [
    	['dc' => 'east', 'use' => 'reporting'],
    	['dc' => 'west'],
    	[]
    ]
];

$m = new MongoClient("mongodb://localhost/", $options);
?>

Configuration is backwards compatible, so users can continue to use old string syntax.
If not necessary, it can be removed.

[alcaeus]

@MarioCuljak note that the CI pipeline doesn't work anymore, as GitHub Actions no longer supports Ubuntu 18.04. If you want to try and update the workflows to use Ubuntu 20.04, feel free to give this a shot in a separate PR. I'm not sure when I'll have the time to look into this.

[MarioCuljak]

@MarioCuljak note that the CI pipeline doesn't work anymore, as GitHub Actions no longer supports Ubuntu 18.04. If you want to try and update the workflows to use Ubuntu 20.04, feel free to give this a shot in a separate PR. I'm not sure when I'll have the time to look into this.

No problem, I'll take a look.