This commit introduces an opaque `otCliInterpreter` type and a set of
new public C CLI APIs (e.g., `otCliInterpreterInit()`,
`otCliInterpreterInputLine()`) to support multiple, dynamically
allocated CLI interpreters per OpenThread instance.
This architecture allows applications to instantiate and manage
multiple concurrent CLI sessions. Backward compatibility is preserved
by retaining the original `otCli*` APIs, which now interact with a
single built-in static interpreter.
The `OPENTHREAD_CONFIG_CLI_STATIC_INTERPRETER_ENABLE` configuration
is also added. It enables support for the static interpreter and is
enabled by default. It can be disabled to save RAM in deployments
that solely use the multi-interpreter APIs.
Abtin Keshavarzian