report: add support for signing reports via varlink backends + make report a varlink...

This adds the following:

1. systemd-report gains a new --sign= option, taking a boolean. If true,
this makes systemd-report generate + systemd-report upload generate a
signed report, instead of a regular one. The signatures are collected
from Varlink-based backends.
2. One such backend is added which does a simple Ed21159 based signing
scheme.
3. this adds a new metrics source which just reports text files
symlinked into a special dir as metrics. This is used to report the
Ed21159 public key as metric, by default, if it exists.
4. finally, systemd-report itself is turned into a varlink service. this
is useful for example for extracting a report from a system coming in
via the varlink/http bridge.

I thought a long time about the format of signing of reports. Initially
i intended to do this like homed's user record signing, i.e. require
normalization of the record, then normalize the record, and write it out
in dense form, since the result. Finally insert the resulting hash into
the user record itself. People have pointed me to the inherent messiness
of signing JSON this way though, as it requires any participant that
wishes to sign/authenticate records this way to implement the exact same
normalization/formatting rules, and in particular in the area of
floating point numbers (of which metrics presumably will have many) this
is quite problematic.

This signing hence goes a different way. instead of expecting
signer+verifier to independently come to the same normalized text form
of the json data, let's instead output a JSON-SEQ sequence, where the
first object is the report, and any subsequent objects are one signature
each. the signatures are supposed to cover the precise binary
representation of the first element in the JSON-SEQ stream. (i.e. from
the RS to the NL).

or in other words: a verifier would receive the JSON-SEQ stream, split
it up before each RS. Then it would leave object 1 unparsed for the
moment, and parse objects 2…n. It would then authenticate object 1's
precise binary representation with objects 2…n. Once that checks out, it
would parse object 1, and use it as report.

diff --cc src/shared/crypto-util.c
Simple merge