Setting environment variables in userdata

[ethanchowell]

/kind feature

Describe the solution you'd like
I'd like to be able to set environment variables for the userdata that is pulling from secrets manager. In our environments we have to access secretsmanager through an HTTP_PROXY and set a few environment variables for things like AWS_CA_BUNDLE and some service endpoints. As is I'm not aware of a way to do this with the existing config.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api-provider-aws version: clusterawsadm version: &version.Info{Major:"2", Minor:"6", GitVersion:"2.6.1", GitCommit:"6db1244a6aea9058cc0546dd7d2151fc2c624acc", GitTreeState:"clean", BuildDate:"2024-07-30T08:09:15Z", GoVersion:"go1.21.12", AwsSdkVersion:"v1.51.17", Compiler:"gc", Platform:"linux/amd64"}
• Kubernetes version: (use kubectl version):

Client Version: v1.27.9
Kustomize Version: v5.0.1
Server Version: v1.30.0

• OS (e.g. from /etc/os-release):

$ cat /etc/os-release 
NAME="Red Hat Enterprise Linux"
VERSION="8.10 (Ootpa)"
ID="rhel"
ID_LIKE="fedora"
VERSION_ID="8.10"
PLATFORM_ID="platform:el8"
PRETTY_NAME="Red Hat Enterprise Linux 8.10 (Ootpa)"

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