feat: client version information

[r10s]

we aim to inform about updates for installations outside of any appstore soon.

there is already a PR on android at deltachat/deltachat-android#4582, however, the information about "update available" is a mockup there.

in general, there are 3 ideas around about how to gather the "update available" infomation - (1) checking a central url, (2) let contacts provide information, (3) let relay provide information. on various one-to-one discussions, outcome is that (3) is the most reasonable way to go.

this PR is about reading update information via IMAP metadata from the relay.

it is up to the UI to call get_app_version() at a reasonable time and frequency, see comment in the code. when called, get_app_version() iterates over all known profiles and relays and checks for version information, returning the newest for the given scope.

we do not use an event, as that is tricky wrt changes - we do not know if other relays report later a newer version. we also do not cache anything, to prevent bad relays avoiding us to update permanently. also it is easier :)

outdated notes and questions

• it is up to the clients to get the needed information, we could let core filter, but it seems easy enough the other way round, and may have debug advantages, one can iterate etc. EDIT: we now filter in core, this also makes the jsonrpc part easier, see review comments

• when is IMAP METADATA actually read? when are they ready? is that really the correct place? i am up to change that, but beware, this is not really my expertise, so someone else may need to take over :)
  EDIT: see below, IMAP METADATA is read on connection, before fetching starts, usually fast enough

• relay part is missing. once the format is settled and discussed shortly here, that should be done soon as well. but this is definitely not my expertise and needed to be done by someone else :)

• key for IMAP METADATA is /shared/vendor/deltachat/appversions - shall we continue use deltachat for compatibility or so? chatmail seems to be more correct
  EDIT: we stay with the current

relay counterpart issue: chatmail/relay#1037

cc @link2xt @Hocuri @hpk42

[Hocuri]

Just some quick comments from quickly reading over it. Would like to finish #8544, afterwards I can take this over if still needed

when is IMAP METADATA actually read?

After starting io, before starting to fetch messages.

when are they ready?

Right now, core doesn't tell the UI that they are ready because the UI never needed any metadata. So, we will need a new event

[link2xt]

key for IMAP METADATA is /shared/vendor/deltachat/appversions - shall we continue use deltachat for compatibility or so? chatmail seems to be more correct

This one is going to contain deltachat APK URLs, so it's fine if it is called deltachat.

[r10s]

This one is going to contain deltachat APK URLs, so it's fine if it is called deltachat.

idea is to have a appversions.json on https://github.com/chatmail/relay that in theory could contain also other app versions, tho not planned for the next months. i was re-using the deltachat namespace only as it was the one used elsewhere, i do not want to complicate things :)
i would tend to stay with that as well tho

[r10s]

i had a call with @Hocuri and incorporated all things we discussed there.

if there are no more comments wrt json format and IMAP METADATA name, i will file an issue for the relays soon.

otherwise, this PR can be re-reviewed, so it gets used by android then soon :)

[hpk42]

overall looks good but the parse-error issue is a blocker i think, and freshness deserves refinement.

[hpk42]

apart from some minor comments/suggestions i am fine enough to approve this :) The corresponding relay-PR chatmail/relay#1038 is merged already.