[Feature] Add HTTP/S proxy support

[criscodex]

Is your feature request related to a problem? Please describe.
Currently, it is not possible to configure astra-cli to connect to Astra DB clusters through an HTTP/s proxy.

Describe the solution you'd like
Add the possibility of setting environment variables or config parameters such as:

HTTPS_PROXY=http://someproxy.local:3088/

in either ~/.astrarc or .env to be picked up by astra-cli

Describe alternatives you've considered
proxychains is an alternative workaround for now: https://github.com/rofl0r/proxychains-ng

Additional context
Certain enterprises secure their Production management hosts by requiring the use of a web proxy to reach the Internet. There is a pressing need to be able to connect to Astra DBs through jumphosts (i.e. bastion servers) where astra-cli is installed.

[criscodex]

@clun this was created following our discussion