Conversions missing for Vec --> Unorm8x4

## Bevy version

0.14

## What you did

I tried to make a custom attribute using the `Unorm8x4` format - but I couldn't find any combination that would compile.

## What went wrong

Passing a `Vec<[u8; 4]>` as the attribute value did not work, the compiler complained that it couldn't convert it.

## Additional information

Looks like the conversion is missing: https://github.com/bevyengine/bevy/blob/main/crates/bevy_render/src/mesh/mesh/conversions.rs#L70-L93

Inspect crates/bevy_render/src/mesh/mesh/conversions.rs around lines 70-93, then reproduce the reported custom-attribute case using a Vec<[u8; 4]> with the Unorm8x4 format. Confirm the missing conversion is supported and verify that the example compiles; done means Vec<[u8; 4]> can be passed as an Unorm8x4 attribute.

Written by the indexing model from the issue text.