rename feature macros

[DenisBiryukov91]

Describe the release item

Currently we are using UNSTABLE to check for unstable feature and SHARED_MEMORY for shared memory feature. These are pretty common names, to avoid collisions with other libraries we need to change them somehow. One proposal is to use schema similar to zenoh pico, where everything is prefixed with Z_FEATURE i.e. this would translate into:
Z_FEATURE_UNSTABLE_API and Z_FEATURE_SHARED_MEMORY.

It is also desirable to make same updated for platform dependent macros, i.e:
TARGET_ARCH_AARCH64 -> Z_TARGET_ARCH_AARCH64.