register_new_matrix_user: Support multiple config files

[V02460]

Support passing multiple config files to register_new_matrix_user. Before, all but the last config file were silently ignored.

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[anoadragon453]

Thanks for tackling this, and sorry for the slow feedback on review.

This looks good to me, other than one small thing below!

[anoadragon453]

Despite Synapse doing this, I think we should raise an error here instead of letting the user go through. Else an action may be carried out that the user didn't intend.

[V02460]

I can’t follow you here. Could you give an example on how that would lead to unintended action?

This suggestion would complicate documenting calls to register_matrix_user in case the config files are auto-generated and possibly empty. Another point is that different behavior between the Synapse CLIs is confusing (and one reason this PR exists).