Implement WMTS REST request encoding

[m-mohr]

See stac-extensions/web-map-links#28

Potential issue:

• OpenLayers can't create a tile matrix set from the URL template and fails => solution: Provide the capabilities document in the href and the URL template (optionally) in the uriTemplate.
• OpenLayers loads the tiles via HTML <img>-tag, which means we can't send any headers to the server

[Copilot]

Pull Request Overview

This PR implements WMTS REST request encoding support for STAC layers. The change adds the ability to handle WMTS links that use REST-style URLs with template variables instead of the traditional KVP (Key-Value Pair) encoding.

• Adds support for WMTS REST request encoding by parsing template variables from link metadata
• Implements variable substitution logic using const, default, or first enum values
• Restructures existing WMTS handling to support both REST and KVP encoding methods

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/ol/layer/STAC.js	Adds REST encoding support with variable substitution and restructures WMTS case handling
CHANGELOG.md	Documents the new WMTS REST request encoding feature

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[Copilot]

Pull Request Overview

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

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