Add support for pnm textures

[luca-della-vedova]

Objective

Add support for the Netpbm image formats, behind a pnm feature flag.

My personal use case for this was robotics applications, with pgm being a popular format used in the field to represent world maps in robots.
I chose the formats and feature name by checking the logic in image.rs

Solution

Quite straightforward, the pnm feature flag already exists in the image crate so it's just creating and exposing a pnm feature flag in the root Cargo.toml and forwarding it through bevy_internal and bevy_render all the way to the image crate.

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Changelog

Added

pnm feature to add support for pam, pbm, pgm and ppm image formats.

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