CRL support

[JeyJeyGao]

As suggested in notaryproject/notation-core-go#125, CRL support is required.

Here is the plan to complete CRL support based on priority:

1. Basic CRL support (v1.3.0): This includes base CRL downloading and validation, with a fallback rule to automatically choose OCSP and fallback to CRL.
2. CRL cache support (v1.3.0): Since CRL is a static file with relatively long-term validity, and a well-known CA's CRL is usually greater than 600KiB, a cache mechanism can significantly improve performance.

@JeyJeyGao Could you move this issue to notation-go? As it's more a library issue than an external user facing one.

[priteshbandi]

LGTM; Additionally, it would be good if we could implement a caching system that allows users to bring their own cache implementation. This would enable users to leverage in-memory caching (for libraries), disk-based caching (for binaries), or any other remote storage option for the cache. Providing this flexibility would give users more control over how the caching is handled in the system and at the same time reducing complexity from our code.

[shizhMSFT]

Cache is important and @JeyJeyGao will share the detailed design later.

[JeyJeyGao]

We will use #425 (for notation-go) and notaryproject/notation-core-go#125 (for notation-core-go) to track the CRL support of notation v1.3.0. Other future work items will be tracked in #428