common configuration file for swh graph rpc-serve, compress, …

swh graph compress has a configuration file (specified as swh -C conf.yml graph compress), but other sub-commands, and in particular rpc-serve don't use it and hard-code options instead. We should have a common approach to configuring swh graph features, and possibly sharing commons settings and defaults (e.g., max_ram and java_tool_options) among different sub-commands.

For reference, what compress uses is in webgraph.py, function check_config.

All configuration file keys are optional, so one doesn't actually need a conffile, but here's an example with some overrides:

graph:
  compress:
    batch_size: 1000
    java_tool_options: -Dlogback.configurationFile={logback}

As you can see interpolation is also supported.

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Migrated from T2072 (view on Phabricator)