core: Set version manually via CustomVersion (#5072)

* Allow version to be set manually

When Caddy is built from a release tarball (as downloaded from GitHub),
`caddy version` returns an empty string. This causes confusion for
downstream packagers.

With this commit, VersionString can be set with eg.
  go build (...) -ldflags '-X (...).VersionString=v1.2.3'
Then the short form version will be "v1.2.3", and the full version
string will begin with "v1.2.3 ".

* Prefer embedded version, then CustomVersion

* Prefer "unknown" for full version over empty

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

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