Allow wp_remote_get args to be filtered for oEmbed calls

[leewillis77]

Currently, the oEmbed code use wp_remote_get() to communicate with the oEmbedded service. However, there's no way to filter the arguments that get used - it simply uses the system wide defaults.

Filtering the defaults (E.g. http_request_timeout) seems overkill when you know that you only want to change things for something specific like an oEmbed call that you know may take longer than 5 seconds (E.g.
​https://github.com/leewillis77/wp-github-oembed/blob/1.2/github-api.php#L35)

The attached patch adds a new filter oembed_remote_get_args which allows plugins to override the wp_remote_get args for oembed calls (Including discovery)

[leewillis77]

Patch

[leewillis77]

The previous patch didn't apply against current trunk, updated patch attached

[nacin]

These filters will need documentation. They could probably benefit from additional context, such as the URL being passed and anything else available to it. Looks good, otherwise.

[DrewAPicture]

Filter docs

[DrewAPicture]

23442.2.diff​ adds filter docs, passing $url per comment:2.

My only real concern there is that $provider_url_with_args passed to the duplicate filter evaluation would differ in form from $url. I suppose we could pass a context string.

[wonderboymusic]

In 28815:

Add a filter/docs: 'oembed_remote_get_args' to WP_oEmbed->discover() and ->_fetch_with_format().

Props leewillis77, DrewAPicture.
Fixes #23442.