Bokeh session ID options

Could we add options to control Bokeh [session ID settings?](https://bokeh.pydata.org/en/latest/docs/reference/command/subcommands/serve.html#session-id-options)

The `bokeh serve --session-ids` command allows "no setting," `unsigned`, `signed`, and `external-signed` values. The `serve` subcommand maps these to `Server` arguments for `sign_sessions` and `generate_session_ids`[here.](https://github.com/bokeh/bokeh/blob/master/bokeh/command/subcommands/serve.py#L553-L572)

If it is somehow important to have Dask options passed along to Bokeh look like options to `bokeh serve` then the same logic has to be ported over. If that is not considered very important then the `sign_sessions` and `generate_session_ids` options could be exposed directly. The only issue is the environment-variable controlled option for `sign_sessions` that Bokeh supports.

Either way I'd be happy to try to put this together as a PR if it makes sense.