Allow specification of generation time when writing parameters

[merose]

This is related to the Yamcs enhancement in yamcs/yamcs#605.

It is currently possible to specify the generation time when writing parameters using the batchSetParameterValues API. The corresponding Python API should also support an optional argument to specify the generation time.

[fqqb]

Addressed as of v1.7.6

Example usage:

from datetime import datetime, timezone
from yamcs.tmtc.model import ValueUpdate

gentime = datetime.now(tz=timezone.utc)

# Example 1
processor.set_parameter_value("/some/parameter", 123, generation_time=gentime)
processor.set_parameter_value("/some/other/parameter", 456, generation_time=gentime)

# Example 2
processor.set_parameter_values({
    "/some/parameter": 123,
    "/some/other/parameter": 456
}, generation_time=gentime)

# Example 3
processor.set_parameter_values({
    "/some/parameter": ValueUpdate(123, generation_time=gentime),
    "/some/other/parameter": ValueUpdate(456, generation_time=gentime)
})

Works also for set_parameter_value because it gets redirected to set_parameter_values (thinking to get rid of the non-batch HTTP method on Yamcs).