Transaction: non-p2p support for tokens

We extract the need-token key from the summary and if set we
calculate a token to use for the operation, which we then pass
to install/update.

For now the actual token just comes from the FLATPAK_TEST_TOKEN
environment var. The details of this will be fleshed out later.

Additionally, this does not support the p2p case, because there
we need the token in order to request the commit during the resolve.
This will also be added later.
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