Add support for Nullable injection types as 'optional" injections

[ggleyzer]

In a number of container-based execution scenarios there appears to be a very useful pattern of declaring an optional injectable resource via Nullable types.

JK used this pattern to declare optional overrides for DB model size thresholds:

    @Inject(ConfigSmallModelMaxBytes)  Int? smallModelBytesMax;
    @Inject(ConfigSmallModelMaxFiles)  Int? smallModelFilesMax;
    @Inject(ConfigMediumModelMaxBytes) Int? mediumModelBytesMax;
    @Inject(ConfigMediumModelMaxFiles) Int? mediumModelFilesMax;

and to declare optional tests in xunit engine:

        @Inject(discovery.ConfigDiscoveryTest)        List<String>? tests;
        @Inject(discovery.ConfigDiscoveryVerbose)     String?       verboseConfig;

Both scenarios were hand coded in the BasicResourceProvider

Another use of that patter can be found in the platform repository, where we "optionally" inject Authenticator and Broker resources.

This change takes a systematic approach for this pattern; all OOTB ResourceProvider implementations allow any resource to be optionally injected using a Nullable type.

Corresponding changes will also be made in the platform code.

While working on this, I discovered and fixed a couple of bugs in the native support for Type.x