`synapse.config.__main__` has the possibility to read a config item. This can be used to conveniently also validate the config is valid before trying to start Synapse.
The "read" command broke in https://github.com/matrix-org/synapse/pull/10916 as it now requires passing in "server.server_name" for example.
Also made the read command optional so one can just call this with just the confirm file reference and get a "Config parses OK" if things are ok.
Signed-off-by: Jason Robinson <jasonr@matrix.org>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>
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