Extend api with `canardRxGetSubscription `

[serges147]

c++ libcyphal needs api to detect already existing subscription (without "touching" one if any). Here is proposal:

/// This function allows to check the effect of canardRxSubscribe() and canardRxUnsubscribe().
/// The return value is 1 if the specified subscription exists, 0 otherwise.
/// The return value is a negated invalid argument error if any of the input arguments are invalid.
int8_t canardRxGetSubscription(CanardInstance* const       ins,
                               const CanardTransferKind    transfer_kind,
                               const CanardPortID          port_id,
                               CanardRxSubscription* const out_subscription);

[serges147]

Looks like canardRxHasSubscription() → bool is enough - we only need to check for possible double subscription for the same kind & port pair.

[pavel-kirienko]

I actually wanted to mention this earlier but forgot: since we'll have to find that specific subscription in the tree anyway, should we not keep canardRxGetSubscription as it is more versatile? There are sensible use cases where one might want to find an existing subscription to modify it.

[serges147]

ok, I revert back to → canardRxGetSubscription