Feature: Added site to Plugin options

[ricardomarsanc]

Context

After trying to use this plugin to integrate Datadog into my Docusaurus site, I run into a permission issue due to my Datadog instance to be in Europe instead of the States. That means I have to send the proper site variable, which is "datadoghq.eu" for Europe instances, instead of "datadoghq.com".

This PR aims to allow the site variable to be sent as another option for the plugin, allowing users to configure the integration properly regardless of where their Datadog instance is based.

Changes

• Added site to PluginOptions (string)
• Send the site variable when initializing Datadog instead of the default "datadoghq.com"
• Added a new test to check that the site variable is being received correctly when sent.
• Added another extra test to check that when no optional options are sent, the result has the correct default values 👍🏽

The new site variable falls back to "datadoghq.com", so no need to pass it in case you don't need a specific one. This makes this change non-breaking in case existing users upgrade to newer versions.

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