* [HUDI-2154] Add index key field to HoodieKey
* [HUDI-2157] Add the bucket index and its read/write implemention of Spark engine.
* revert HUDI-2154 add index key field to HoodieKey
* fix all comments and introduce a new tricky way to get index key at runtime
support double insert for bucket index
* revert spark read optimizer based on bucket index
* add the storage layout
* index tag, hash function and add ut
* fix ut
* address partial comments
* Code review feedback
* add layout config and docs
* fix ut
* rename hoodie.layout and rebase master
Co-authored-by: Vinoth Chandar <vinoth@apache.org>