Support absolute path for Spin component source files

Currently, components can only access files relative to the Spin.toml of an app (see docs).

For example if i wanted a module to read cpu temp /sys/class/thermal/thermal_zone0/temp from inside a spin app, I get a source paths must be relative error. However, if i directly mount the file's directory with the --direct-mounts flag, it works. It seems weird that it can't copy a file but it can mount it. I would like to be able to read a file at an absolute path without being able to modify it (as with --direct-mounts).

Alternatives

Continue to only support relative to prevent exposing sensitive info. "If you constrain relative paths to the app directory then they are much less likely to be able to access sensitive info. /etc/passwd is actually a bad example these days, but there are often user credentials in ~/.whatever" - @lann.
This brings up the point that maybe we need some way to express whether a component should get access to privileged information rather than inferring what could or could not be privileged based on file path.

Start with the Spin configuration documentation and the existing --direct-mounts behavior described in the issue. Clarify the security and read-only access requirements for absolute source paths; done means a component can read the requested absolute file without modification while preserving protection against sensitive paths.