inlined data: embed small object to the metadata directly

[iwanbk]

• define the size that considered small: pingcap Titan and fjall consider 1KB is a large value. Garage use 4KB as large value. We will start with 1KB and provide the command line config to change it
• performance improvement: when getting a key, we have to get the metadata first, which means two gets. with inlined data, we only need one get
• write reduction: doesn't need to write to the disk storage-> one less write
• see garage implementation

We could do it after finishing the sled to fjall migration
deliverables:

• of course the code
• configurable from the command line:
  - the max size of the object to be inlined
  - option to enable fjall key-value separation, see : fjall: key value separation #40 (comment)
• test code
• benchmark code, possibly using cargo bench?

Tasks:

• prepare the metadata to support embedded/inlined data re-structure metadata to support multiple type #64
• Implement the inlined data on single object. Only single object because multipart is usually for the bigger one. add support for inlined data on metadata #66
• benchmark code
• add key-value separation optional feature
• implement the inlined data on multipart(if needed), require key-value separation feature.

[iwanbk]

Ref:

https://github.com/deuxfleurs-org/garage/blob/d3226bfa91d4500063c5c287c6256729dcbb3f88/src/api/s3/put.rs#L134-L174