refactor(ec2): avoid logging somewhat sensitive data (aws#6149)

## Problem
We currently log `STREAM_URL` and `TOKEN` which are used to establish
the SSM session. However, these could be used to establish a connection
outside the toolkit.

## Solution
- Omit these from the logs. 
- Also add a logging statement to make it easier to find these
`connect_script` logs.

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).

License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Hweinstock

packages/core/resources/ec2_connect

@@ -47,8 +47,8 @@ _main() {
 
     _require AWS_SSM_CLI "${AWS_SSM_CLI:-}"
     _require AWS_REGION "${AWS_REGION:-}"
-    _require STREAM_URL "${STREAM_URL:-}"
-    _require TOKEN "${TOKEN:-}"
+    _require_nolog STREAM_URL "${STREAM_URL:-}"
+    _require_nolog TOKEN "${TOKEN:-}"
     _require SESSION_ID "${SESSION_ID:-}"
     _require LOG_FILE_LOCATION "${LOG_FILE_LOCATION:-}"
 

packages/core/src/awsService/ec2/model.ts

@@ -242,6 +242,7 @@ export class Ec2Connecter implements vscode.Disposable {
         const ssmSession = await this.startSSMSession(selection.instanceId)
 
         const vars = getEc2SsmEnv(selection, ssm, ssmSession)
+        getLogger().info(`ec2: connect script logs at ${vars.LOG_FILE_LOCATION}`)
         const envProvider = async () => {
             return { [sshAgentSocketVariable]: await startSshAgent(), ...vars }
         }