Add (3rd party board): DPTechnics Walter board support

[vThibo]

Description of Change

This PR adds board definitions for DPTechnics Walter, a compact IoT development board combining an ESP32-S3 with a Sequans Monarch 2 GM02SP cellular modem.

Related links

More information about Walter can be found on its CrowdSupply page:
https://www.crowdsupply.com/dptechnics/walter

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Warnings

⚠️

Some issues found for the commit messages in this PR: the commit message "Add Walter board support": summary looks empty type/action looks empty Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert,test sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).

👋 Hello vThibo, we appreciate your contribution to this project!

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