Allow configuration of the oEmbed URLs. (#10714)

This adds configuration options (under an `oembed` section) to
configure which URLs are matched to use oEmbed for URL
previews.

docs/sample_config.yaml

@@ -1075,6 +1075,27 @@ url_preview_accept_language:
 #   - en
 
 
+# oEmbed allows for easier embedding content from a website. It can be
+# used for generating URLs previews of services which support it.
+#
+oembed:
+  # A default list of oEmbed providers is included with Synapse.
+  #
+  # Uncomment the following to disable using these default oEmbed URLs.
+  # Defaults to 'false'.
+  #
+  #disable_default_providers: true
+
+  # Additional files with oEmbed configuration (each should be in the
+  # form of providers.json).
+  #
+  # By default, this list is empty (so only the default providers.json
+  # is used).
+  #
+  #additional_providers:
+  #  - oembed/my_providers.json
+
+
 ## Captcha ##
 # See docs/CAPTCHA_SETUP.md for full details of configuring this.