feat: add permissions management

[Garcia6l20]

This PR adds support for configuring file and directory permissions in cargo-packager.

Users can explicitly define Unix-style permissions (owner, group, other) for packaged assets, giving finer control over how files are installed on target systems.

Example:

[[package.metadata.packager.resources]]
src = "private-key.der"
target = "data/private-key.der"
permissions = 0o600

# or noisy:
[[package.metadata.packager.resources]]
src = "cert.der"
target = "data/cert.der"
permissions = { group = { read = true, write = false }, other = { read = false } }