feat: allow users to provide an existing self-signed certificate

[reubenmiller]

Allow users to provide an existing certificate that should be used for the device to support use-cases such as interacting with a PKCS11 module

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
26	0	0	26	100	13.041389s

Passed Tests

Name	⏱️ Duration	Suite
Get IoT Url	0.599 s	Account
Get Account ID	0.330 s	Account
Create certificate key pair	0.507 s	Certificates
Create CSR using AWS CA	0.473 s	Certificates
Upload self-signed certificate	0.369 s	Certificates
Create Certificate and Thing	0.679 s	Composite
Start/stop MQTT logger	0.619 s	Logger
Assert message count	2.770 s	Logger
Assert MQTT message	2.700 s	Logger
Create a new Policy	0.177 s	Policies
Deleting a non-existent Policy should not throw an error	0.082 s	Policies
Deleting a non-existent Policy should throw an error	0.079 s	Policies
Assert that a policy exists	0.168 s	Policies
Fails if policy does not exist	0.080 s	Policies
Create Thing with auto cleanup	0.158 s	Thing
Create Thing with Principal	0.592 s	Thing
Delete Thing	0.244 s	Thing
Delete non-existent thing	0.078 s	Thing
Check if thing exists	0.078 s	Thing
Throws an error if thing exists	2.252 s	Thing
Get Local Command Topic	0.001 s	Topics
Get Cloud Command Topic	0.001 s	Topics
Get Telemetry Topic	0.001 s	Topics
Get Registration Topic	0.001 s	Topics
Get cloud topic for the AWS shadow	0.001 s	Topics
Get local topic for the AWS shadow	0.001 s	Topics