[WebGPU] Give BindGroupLayouts a notion of external textures (like BindGroups have)
<rdar://problem/109585492>
Pull request: https://github.com/WebKit/WebKit/pull/14107
Committed 264437@main (936c42734f58): <https://commits.webkit.org/264437@main> Reviewed commits have been landed. Closing PR #14107 and removing active labels.