[Impeller] Add a flat VertexAttributeFormat for vertex inputs

[bdero]

Related to #186309.

Adds a VertexAttributeFormat enum and derives it from a reflected ShaderStageIOSlot, then routes the Metal, Vulkan, and GLES backends through it when building their native vertex input format. This replaces three independent switches over the slot's scalar type, bit width, and component count with one shared vocabulary, which is the foundation for the vertex format expansion tracked in the issue.

This is a behavior preserving refactor. Only the formats reflection produces today are reachable, and each backend keeps its existing support set, so GLES still rejects half-float and 32-bit integer attributes while Metal and Vulkan accept them. The one intentional cleanup is that boolean vertex inputs, which are not expressible in a vertex stage, now map to invalid on every backend instead of the previous inconsistent mappings.

New tests cover the format derivation across scalar kinds, component counts, bit widths, and matrix inputs, plus the GLES mapping including the half-float and 32-bit integer formats it rejects.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request introduces a unified VertexAttributeFormat enum and a GetVertexAttributeFormat helper on ShaderStageIOSlot to centralize vertex attribute format derivation. The GLES, Metal, and Vulkan backends are updated to use this new format, simplifying their vertex format mapping logic, and corresponding unit tests are added. Feedback suggests capturing vec_size by value instead of capturing this in the lambda within GetVertexAttributeFormat to avoid potential compilation issues on older compilers in a constexpr context.

[gaaclarke]

LGTM, thanks!

[gaaclarke]

@walley892 I'm confused, I can't remember if bdero needs 2 reviews or not. If so can you please provide a second review. LGTM.

[bdero]

I'm confused, I can't remember if bdero needs 2 reviews or not. If so can you please provide a second review. LGTM.

I only need one. :)