[IFRT] Rename `xla::ifrt::IoCallable` to `xla::ifrt::CustomCall`
[IFRT] Rename `xla::ifrt::IoCallable` to `xla::ifrt::CustomCall`

Based on a discussion on the expected semantics of "I/O callable", we change
its name to "IFRT custom call" ("IfrtCustomCall" wherever a single word is
preferred) in the IFRT API level. Its C++ type name will use
`xla::ifrt::CustomCall` as the prefix instead of `xla::ifrt::IoCallable`.

A high-level API that uses IFRT custom call may use a different name depending
on what features they offer.
