This commit introduces the `BorderAgent::EphemeralKeyManager` class,
which manages the use of the ephemeral key by the Border Agent.
The `EphemeralKeyManager` uses its own DTLS transport and CoAP secure
sessions. This allows the `EphemeralKeyManager` and the `BorderAgent`
service (which uses PSKc) to be enabled and used in parallel.
Previously, a single transport and session was shared between these
functions, requiring the normal BA service (with PSKc) to be stopped
before the ephemeral key could be used.
This is a fundamental change and improvement to the ephemeral key and
Border Agent functionality. Therefore some existing `otBorderAgent`
APIs need to be modified. For example, `otBorderAgentGetState()`,
which returned the Border Agent state to indicate whether there were
any active sessions, is no longer meaningful, as different
sessions/transports are now used for PSKc and ephemeral key, and
there can be multiple sessions. This commit intentionally renames and
changes the `otBorderAgent` public APIs, specifically all those
related to ephemeral key use, to highlight the fundamental change in
behavior. While this can cause backward incompatibility, it requires
app layer code that used the previous APIs to be updated to take into
account the new behavior.
This commit also updates `nexus/test_border_agent`, adding new tests
to validate the new behavior (e.g., BA service and ephemeral key
parallel sessions). It also includes and validates the Border Agent
counter updates under different scenarios (this enhances and replaces
`test_ephemeral_key_counters.py`).
Abtin Keshavarzian
bdb394eb3b