### Summary
Integration with rockset/rocksdb-cloud to add CloudEnv features for operating with cloud services like AWS S3 as a datastore.
- Created a sub-crate `librocksdb_cloud_sys` which contains the `rocksdb-cloud` sub-module containing code for managing cloud interactions, extracted from rockset/rocksdb-cloud and currently residing in [tikv/rocksdb~6.4.cloud](https://github.com/tikv/rocksdb/tree/6.4.cloud)
- Made necessary changes to [tikv/rocksdb~6.4.tikv](https://github.com/tikv/rocksdb/tree/6.4.tikv) that is housed in the sub-module `rocksdb` within the parent crate.
- Wrote CMakeLists.txt for use in the build process of `rocksdb_cloud` as a static library, usable for C-ABI based FFI calls. Added the above code to the build process of parent crate `librocksdb_sys` along with C-ABI binding code within `crocksdb`
- Created 'cloud.rs' and added `new_aws_env()` method to Env type of `rocksdb.rs` that merely makes a call to the `CloudEnv::NewAwsEnv()` via FFI for use as library features upstream. A test for the same was also added here.
- Used feature based conditional compile macros to all cloud interaction code.
- Made requisite changes to CI config for handling AWS SDK compilation.
- Added a sequence of steps to update submodule within Makefile

Solves #514

## Checkpoints

- [x] Added rocksdb-cloud as submodule in crate `librocksdb_cloud_sys`.
- [x] Write CMakeLists.txt to build only `cloud` code.
- [x] Generate C-bindings to interface with `NewAwsEnv` and `CloudEnvOptions`
- [x] Create `Env::new_aws_env` as an interface in rust
Signed-off-by: Devdutt Shenoi <devdutt@outlook.in>