fix: only initialize session when used

[reubenmiller]

Prevent class instantiation problems when the AWS config is not set by lazily creating the session when trying to access the AWS API

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
15	0	0	15	100	2.839891s

Passed Tests

Name	⏱️ Duration	Suite
Get IoT Url	0.406 s	Account
Get Account ID	0.330 s	Account
Create certificate key pair	0.286 s	Certificates
Create CSR using AWS CA	0.333 s	Certificates
Upload self-signed certificate	0.349 s	Certificates
Create a new Policy	0.153 s	Policies
Deleting a non-existent Policy should not throw an error	0.071 s	Policies
Deleting a non-existent Policy should throw an error	0.068 s	Policies
Assert that a policy exists	0.150 s	Policies
Fails if policy does not exist	0.067 s	Policies
Create Thing with auto cleanup	0.142 s	Thing
Delete Thing	0.211 s	Thing
Delete non-existent thing	0.068 s	Thing
Check if thing exists	0.067 s	Thing
Throws an error if thing exists	0.139 s	Thing