Add specification proposal for maxcomponent and mincomponent nodes

[chinmaychahar]

Summary

Add specification proposal for maxcomponent and mincomponent nodes to the Math Nodes section of the Specification Proposals document.

Closes #2802

Description

Per the discussion in #2802 with @jstone-lucasfilm :

• These nodes support all vectorN and colorN types (vector2, vector3, vector4, color3,
  color4)
• Output type is float

The proposal format follows existing nodes such as magnitude and dotproduct in the Standard Nodes document.

Use Cases

As mentioned in the issue, two places in the MaterialX data libraries currently use verbose 5-node patterns (3 extract + 2 max/min) that these nodes would simplify to a single node:

1. Sheen intensity for glTF PBR (libraries/bxdf/gltf_pbr.mtlx) — uses maxcomponent pattern
2. Absorption coefficient for OpenPBR Surface (libraries/bxdf/open_pbr_surface.mtlx) — uses mincomponent pattern

Scope

This PR addresses the specification proposal only. I can work on the follow-up implementation via graph definitions as a
separate PR once the proposal is reviewed and merged.

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[jstone-lucasfilm]

Great work, @chinmaychahar, and this proposal looks good to me!

[chinmaychahar]

Thanks @jstone-lucasfilm ! Should I proceed with the implementation on the same PR or would you prefer I open another one?

[jstone-lucasfilm]

Let's merge this update to the proposals document, and you can then proceed with an implementation in a separate PR.

The update to the proposals document should be safe to include in our v1.39.5 release, which is currently in testing, while the actual graph definition of the node would likely land in our next release.