Add "wait for trigger" flag for trigger based streaming

[guruofquality]

SOAPY_SDR_TRIGGER could be used in the activateStream() call to indicate a wait not on time, but on a external trigger event for receive samples. Similarly SOAPY_SDR_TRIGGER could be used in writeStream() to indicate that the transmission should wait on a trigger as well.

Its up to the individual module to handle the flag or not. Some implementations might encode the trigger request into the rx control packet or the tx data packet. Some might setup a one-time trigger through board registers, etc..

[guruofquality]

b03c484