F http proxy

Summary

Adds support for http proxies (as per https://gitlab.indiscale.com/caosdb/customers/aba/management/-/issues/32).

Focus

1. Support for http proxies via the new https_proxy and http_proxy parameters and config options and by respecting the widely used HTTPS_PROXY and HTTP_PROXY environment variables. This deprecates the socket_proxy option because the functionality is contained in the new options.
2. The support for http proxies made it necessary to use the requests library. This is a long overdue refactoring because the old custom implementation for streaming https connections was from the early days of caosdb-pylib when streaming + https was not supported by any standard library back then. This reduces the complexity and uses a well-tested, regularly updated, yet stable library instead.
3. Add dependency on python-dateutil because it is used in high_level_api.py.
4. As a nice side-effect, http connections are possible now (Does anyone still use these?)

Test Environment

Manual testing required!

Test with a local socks5 proxy

1. ssh -D 8888 localhost (mind: if you exit this, the socks proxy is gone...)
2. configure pycaosdb.ini for socks proxy or use the env var
3. try connecting to demo server

Also try to configure a wrong socks proxy (e.g. wrong port) - this should fail.

Test with a local http proxy

1. start dockerized tinyproxy: docker run --rm --name tinyproxy -p 8888:8888 -e "ALLOWED=255.255.255.255/0" ajoergensen/tinyproxy
2. configure pycaosdb.ini for http proxy or use the env var
3. try connecting to demo server

Also try to configure a wrong http proxy.

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