Expose existing boto3 proxy configuration options

**Is your feature request related to a problem? Please describe.**

I have configured the AWS CLI to use a proxy by setting the `HTTPS_PROXY` environment variable. However the proxy presents an internal certificate and I see a message saying "Cannot connect to proxy". (I have verified that the proxy is usable with `curl` by testing that I can connect and that the certificate presented is valid.)

There is no way to configure [boto3's existing `proxy_ca_bundle` option](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html#configuring-proxies), whether by environment variable or command flag.

**Describe the solution you'd like**

- I want the `aws` command to make use of an`AWS_S3_PROXY_CA_BUNDLE` environment variable when it is set
- I want the `s3` subcommand to have an explicit `--proxy_ca_bundle` flag

**Describe alternatives you've considered**
I've tried setting the value in a dictionary in the config file, but it's not working and there's no example to refer to, so I'm not sure if there's a problem with the way I'm specifying the value, or with the value itself, or if this option is not expected to work.

**Additional context**
The proxy restricts egress traffic to expected S3 buckets in order to prevent compromised applications from exfiltrating data or establishing a command-and-control backchannel (eg SolarWinds and log4shell-style attacks).

I also checked the v2 branch to see if the option was there, but didn't find any indication that this setting is on anyone's radar.