Add flags for `SupportedFeature` and `ExemptFeature` in conformance tests

[aryan9600]

What would you like to be added:
A flag for configuring SupportedFeature and another for ExemptFeature in the conformance test suite.

Why this is needed:
This would let implementations choose which features they want to include in their conformance test runs, depending on which features are supported/unsupported by them.

[gyohuangxin]

I will work on this.
/assign

[gyohuangxin]

@aryan9600 I created a PR to add these flags, please feel free to review and leave comments.