Draft: port to libsoup3 (+ drop deprecated apis, stop using deprecated glib api)

this is a port to the upcoming (will be released later this year) libsoup 3.x

• all deprecated APIs have been dropped (most of them do not work and with libsoup3 a clean API break will be needed anyway)
• since libsoup 3 needs newer glib, i also bumped the glib version - with that, i had to drop use of deprecated glib APIs (mostly GTimeVal and some gobject-related stuff)
• it will probably be necessary to make the libsoup3 version installable alongside the libsoup2 version, so it might be a good idea to rename the library

it'll also be necessary to decide the path going forward with maintenance of the soup2 version

• this needs https://gitlab.com/uhttpmock/uhttpmock/-/merge_requests/12 to be able to run tests - so the concerns in the linked MR will also need solving
• this does not pass all tests for now

WIP:

• async API for download stream
• rework upload stream - i could actually use some advice here - the way the upload stream works appears to be fundamentally incompatible with how libsoup3 works - since it's not possible to call libsoup from several threads anymore (you can only use the thread the session was created in, and all operations run in the thread-default context), and also message i/o being streaming-only now - with the current approach, it runs a separate upload thread and when you write into the stream synchronously, it will wait for that portion of the data being written on the network - i can't think of a good way to implement that behavior...
• that means any tests currently using the upload stream fail
• services still use threads (which is not gonna work; I rewrote the base service code to use libsoup async APIs, the services need to be adapted)

for now i'm opening this to get further advice/feedback