Add `encase` trait implementations

[teoxoy]

What problem does this solve or what need does it fill?

I would like to remove trait implementations on foreign types from encase for the reasons listed in teoxoy/encase#94.

Describe the solution you'd like

Add encase trait implementations behind an encase feature flag.

They are currently hosted here: https://github.com/teoxoy/encase/blob/fcc83d706f4a51bfbe22c9901973f1c2fa806526/src/impls/glam.rs

Describe alternatives you've considered

Status quo.

Additional context

teoxoy/encase#94

[teoxoy]

Before releasing a v0.12 with all optional features removed, I would like to first get a confirmation that glam developers are willing to host the trait implementations.

[bitshifter]

Yes, this would be OK with me. I know it's a problem for bevy every time I make a glam release they need a matching encase release. I could see this being easier the other way around.

It would be good if there was a better way of dealing with this in Rust one day.