docs: what are the values that the `source-type` can have?

[panlinux]

The docs say:

You must specify at least the source key. The source-type key is optional, but recommended, as it is used to specify how the source should be handled.

But it doesn't say what are the possible values that source-type can have.

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Reported from: https://documentation.ubuntu.com/charmcraft/stable/common/craft-parts/reference/plugins/dump_plugin/