H5P: get site UUID using H5P API

MDL-67058 adds a task to install the latest version of the H5P content types. It requires connecting to the H5P API, whose endpoints expects a POST with "application/x-www-form-urlencoded" data, which must at least include the site's UUID.

Although the site's UUID could be generated locally, with the only limitation of not being longer than 36 characters, the correct way to do this is to send a request to the "api.h5p.org/v1/sites" endpoint without any UUID and obtain a unique UUID that will be registered for the site.

The goal of this issue is getting the UUID from the H5P API and use it in any H5P API request. Submission of additional information, although not sensitive, should be acknowledged by a site administrator, and is beyond the scope of this issue.

Source:

• https://h5p.org/comment/11848#comment-11848
• https://h5p.org/comment/21819#comment-21819
• https://h5p.org/tracking-the-usage-of-h5p