Embedded App: do not extract sources

Describe your feature request

Is your feature request related to a problem? Please describe.
The extraction to /tmp/frankenphp_... brings a few issues with it.

• /tmp must be writable.
• systemd ProtectTmp can lead to unexpected paths
• startup extracts the tar, can be slow
• old files linger in /tmp until restart of the machine
• source code is human readable without any reverse engineering

Describe the solution you'd like
Embedded filesystem inside the binary, rather than extracting the tares?

• possibly encrypted? not a goal to truly make it "secure", but not immediately showing the source code is a plus

Describe your approach

• create named virtual filesystem in go
• redirect caddy to use that vfs for e.g. file_server and matchers
• overwrite php file handlers to redirect to the vfs
• work around possible opcache issues (?)
• find a good solution for folders that need write access (e.g. symfony cache)?
• embed a database...? not sure how this could be possible, but who knows. otherwise create a proof of concept using sqlite.

I'll work on this slowly over the next weeks. Don't expect anything immediately. I think @dunglas mentioned that something like this was planned as a potential commercial offering, so if you don't want me to pick it up, just let me know now before I start.