Add ISM APIs to CLI

[dbbaughe]

1. Create policy
2. Add policy
3. Update policy
4. Get policy
5. Remove policy from index
6. Update managed index policy
7. Retry failed index
8. Explain index
9. Delete policy

https://opendistro.github.io/for-elasticsearch-docs/docs/im/ism/api/

[lukajose]

why not just use the api?

[VijayanB]

@lukajose our aim is to provide better user experience while interacting with opendistro plugins. Though it is possible to use API, having this apis integrated as commands in cli will help user not to spend too much time on setting up or building api request. Moreover, user don't have to go to multiple plugin's api documentation to understand api contracts, they can easily perform this as operation in an easy to use interface from command line.