[Feature] <Allow overriding values of allowed parameters at runtime>

[sanyogitaranade]

Is your feature request related to a problem? Please describe.
CHProxy allows to specify a list of query string parameters to send to ClickHouse. But it does not allow overriding a value of those parameters at query time.

Describe the solution you'd like
e.g. if log_comment is allowed to be passed to ClickHouse with a value value1,
when a request is made like
echo "select now()" | curl '<chproxy-host>:9001?user=<chproxy-user>&password=<chproxy-password>&log_comment=newValue1' --data-binary @-

log_comment is value1, instead of newValue1.

The desired behavior would be to

1. allow a set of query string parameters with default values to be sent to ClickHouse
2. if the value is overriden at query time, pass that value, else send default value