payload API for zenoh-c

[milyin]

After discussion which started with @kydos and continued with @Mallets, @DenisBiryukov91. @sashacmc , @milyin the following decisions were made about new payload API for zenoh-c

• z_owned_payload_t is an opaque object which can be received as part of sample or reply (instead of pair (buffer,length) at this moment)
• the access to data in the payload object is perfromed through reader interface, like in Rust. For c the interface is similar to file interface with read(), seek(), rewind(), etc. No direct access to memory is provided
• the set of functions to decode payload into integer, string, double is provided. These functions corresponds to From<Payload> traits in rust
• constructing new payload for sending was not discussed. It should be possible to construct playload from string, integer, double as in Into<Payload> traits in rust. The way to construct payload from arbirtary user data should be provided also. To be decided, how. Write interface for symmetry? Just construct from user-owned buffer? Allocate single zenoh-owned buffer? All these variants?
• It's unclear how z_put() should accept payload object: by taking ownership or by loaning? Accepting it with taking ownership (with z_move) is more convenient, no need to additional drop. But in this case for sending same payload for multiple destinations the payload should support clone operation (rcinc). Should the payload support it?

[milyin]

Additional point: encoder/decoders should be also applicable to values of the attachment. Though this is related to discussion about attachment interface and should be started from Rust. Issue to be created

[milyin]

• the payload object is named z_owned_bytes_t now
• reader inteface for payload is imlemented (z_owned_bytes_reader_t)
• writer/construction interface is to be done (add missing methods to z_bytes_t #371 , add writer operation for z_bytes_t #372 )
• clone operation is implemented: z_bytes_clone