Revert the revert: bring back integration test foundation (#130)

*Issue #, if available:* 

*Description of changes:*
**Background**
The integration test framework was previously reverted due to Docker
build failures. This PR restores the functionality with a fix for the
underlying issue.

The original integration tests used reqwest = "0.12" with default
features, which includes native-tls that depends on system OpenSSL
libraries on Linux for the time being. This causes build failures in
environments without openssl development tooling installed:
```
Could not find directory of OpenSSL installation, and this `-sys` crate cannot proceed without this knowledge. If OpenSSL is installed and this crate had trouble finding it, you can set the `OPENSSL_DIR` environment variable for the
compilation process.
```

Switched `reqwest` from `native-tls` to `rustls-tls`, which uses AWS-LC
under the hood.

```
# Before
reqwest = "0.12"

# After
reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }
```

**Additional Changes**
* Added CDLA-Permissive-2.0 for webpki-roots dependency
* Added PR triggers to Docker workflow to catch build issues before
merge

Testing
* Local builds (macOS, Linux)
* Docker builds (linux/amd64, linux/arm64)
* Integration tests functionality preserved
* License compliance checks pass

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: reyhankoyun

.github/workflows/docker.yml

@@ -2,7 +2,9 @@ name: "Docker Image CI"
 
 on:
   push:
-    branches: [main]
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
 
 jobs:
   build: