New CEL convenience function: random()

*Title*: *New CEL convenience function: random()*

*Description*:
* When called, use `Random::RandomGenerator` to generate a random uint64
* Useful in [Access Logger Filter](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/access_loggers/filters/cel/v3/cel.proto#extensions-access-loggers-filters-cel-v3-expressionfilter) to introduce sampling
* Although this is possible with the Runtime filter, this would be useful for end users

*Questions*:
* Should this convenience function be available everywhere? E.g. [the access log formatter](https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/formatter/cel/v3/cel.proto#extensions-formatter-cel-v3-cel)?
* On the flip note, should this function, and others in the future, be explicitly requested by the user? E.g. a list of extensions provided with a cel expression under a new type:
```
message CelActivationSettings {
repeated google.protobuf.Any typed_functions = 1;
}

message CelRandomFunction {
}

...
```