mod_external_services: Validate services added via events While writing developer documentation it became obvious that i was silly to have one item format for config and items API, and another format for the event API. Then there's the stanza format, but that's a common pattern. This change reduces the possible input formats to two and allows other modules the benefit of the processing and validation performed on items from the config.

mod_external_services: Allow specifying a credential generation callback This is especially targeted at services added via the items API. More involved credential generation should use the event hook.

mod_external_services: Prepare to allow more credential algorithms Not sure what algorithms might fit here. Separation makes some sense. This is also a preparation for having a callback. (See next commit)

mod_external_services: Support adding services via items API

mod_external_services: XEP-0215: External Service Discovery

mod_component: Return extended error condition when not connected This might be something to write a XEP about.

mod_component: Reply with a different error when not connected The wait, service-unavailable is overloaded by XEP-0045 to mean that the room has reached the maximum number of users. See #1495. Bouncing errors for components is tricky since there is no way to tell that it comes from the server hosting the component, not from the other end of the component connection.

net.http.parser: Allow configuration of the chunk size fed to the parser

net.http.parser: Switch tests so that CRLF conversion of input data is optional

net.http.parser: Fix incorrect path in test