remove internal APIs from public interface

[milyin]

Describe the release item

We should not expose internal APIs or at least should explicitly mark them as internal in the documentation (e.g. APIs needed for plugin support only)

The list of APIs to be checked

keyexpr -> get_nonwild_prefix keyexpr -> is_wild keyexpr -> chunks keyexpr -> strip_prefix

publication.rs: Priority::MIN, Priority::MAX, Priority::NUM

WhatAmI matcher

ZenohId -> size, max_size, rand, into_keyexpr, to_le_bytes

[milyin]

Consider using #[doc(hidden)] attribute for them

[wyfo]

WhatAmIMatcher is used as argument for zenoh::scout, so I think it can be removed from the list?

[milyin]

All functionality which is needed by some external crates but should not be part of public API should be moved into traits available through "internal" mod. Here is an example: https://github.com/eclipse-zenoh/zenoh/pull/1046

[milyin]

from @Mallets:

• [x] buffers goes to internal
• [x] ZBytes impl for ZSlice and ZBuf goes to internal
• [x] zenoh::init goes to internal
• [x] plugins goes to internal
• [x] runtime goes to unstable and internal

[milyin]

All functionality which is needed by some external crates but should not be part of public API should be moved into traits available through "internal" mod. Here is an example: #1046

Seems that #[cfg(all(feature = "unstable", feature="internal"))] is enough, no need to complicate with traits

[wyfo]

Instead of #[cfg(all(feature = "unstable", feature="internal"))], I think we should annotate unstable items used internally with #[cfg(any(feature = "unstable", feature="internal"))]. It would allow to use "internal" items without needing to build with unstable feature. (Of course, zenoh::internal module would only have #[cfg(feature = "internal)])