cfsignal: add proxy support

[jpculp]

Description of changes:

Adds support for sending signals to CloudFormation through a set proxy.

Testing done:

Launched an aws-ecs-2 node with the following userdata:

[settings.ecs]
cluster = "<MY_CLUSTER>"

[settings.network]
https-proxy = "<MY_PROXY>:8888"

[settings.cloudformation]
should-signal = true
stack-name = "<MY_STACK>"
logical-resource-id = "<MY_RESOURCE>"

Status of cfsignal.service:

bash-5.1# systemctl status cfsignal
○ cfsignal.service - Send signal to CloudFormation Stack
     Loaded: loaded (/aarch64-bottlerocket-linux-gnu/sys-root/usr/lib/systemd/system/cfsignal.service; enabled; preset: enabled)
     Active: inactive (dead) since Fri 2024-11-01 00:19:10 UTC; 6min ago
   Duration: 2.483s
   Main PID: 1157 (code=exited, status=0/SUCCESS)
        CPU: 25ms

Nov 01 00:19:07 localhost systemd[1]: Started Send signal to CloudFormation Stack.
Nov 01 00:19:09 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:09 [INFO] System status is: running [0]
Nov 01 00:19:09 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:09 [INFO] Connecting to IMDS
Nov 01 00:19:10 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:10 [INFO] Received meta-data/instance-id
Nov 01 00:19:10 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:10 [INFO] Received dynamic/instance-identity/document
Nov 01 00:19:10 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:10 [INFO] Region: "us-west-2" - InstanceID: "<MY_INSTANCE>" - Signal: "SUCCESS"
Nov 01 00:19:10 ip-XXX-XXX-XX-XX.us-west-2.compute.internal cfsignal[1157]: 00:19:10 [INFO] lazy_load_identity;
Nov 01 00:19:10 ip-XXX-XXX-XX-XX.us-west-2.compute.internal systemd[1]: cfsignal.service: Deactivated successfully.

Excerpts from CloudTrail:

"eventName": "SignalResource",
"eventSource": "cloudformation.amazonaws.com",
"eventTime": "2024-11-01T00:19:10Z",
"eventType": "AwsApiCall",
"eventVersion": "1.08",
"managementEvent": true,
"readOnly": false,

...

"requestParameters": {
    "logicalResourceId": "<MY_RESOURCE>",
    "stackName": "<MY_STACK>",
    "status": "SUCCESS",
    "uniqueId": "<MY_INSTANCE>"
},

...

"sourceIPAddress": "<MY_PROXY>",
"userAgent": "aws-sdk-rust/1.3.3 os/linux lang/rust/1.81.0",

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[vyaghras]

🚢

[cbgbt]

style nit: match statements like this are more idiomatically written using Option::map.

[jpculp]

Suggested change

	let https_proxy: Option<String> = match env::var_os("HTTPS_PROXY") {
	let https_proxy = env::var("HTTPS_PROXY").ok();
	let no_proxy = env::var("NO_PROXY")
	.ok()
	.map(|n| n.split(',').map(|s| s.to_string()).collect::<Vec<_>>());

So in practice like this. It does look clean, but since we already have approvals I think we can save it for next time we're in the crate or if clippy flags it.