Implement setting prefixes to scope a setting to a node type

After #3510 and #3739 we propagate ALTER ROLE..SET commands to all nodes by default. In some cases, it would be useful to scope a setting to a particular type of node. The GUC syntax is relatively flexible, so we could add a prefix to the configuration name and when propagating/applying the setting use the actual name.

For instance, we could differentiate between coordinators and workers are physical nodes:

- SET setting - apply globally
- SET coordinator.setting - apply only on coordinator(s)
- SET workers.setting - apply only on workers

It may also be useful to differentiate between settings for planning and executing distributed queries vs. queries on shards using prefixes.

These mechanics should eventually apply across all different SET capabilities (SET, SET LOCAL, ALTER ROLE .. SET, ALTER DATABASE .. SET).