add MAINTAINERS.md file to each repository in Spin project

[michelleN]

The governance document was just merged and requires a MAINTAINERS.md file in each repo:

New maintainers MUST be nominated by existing maintainers. Maintainers are to discuss and agree in a private setting adding a new maintainer. Once a descision has been made, a maintainer may be added to the project via a pull request to the relevant MAINTAINERS.md file.

This issue tracks the work to make that change.

[michelleN]

Core Spin:

• Spin: add MAINTAINERS.md #2683

SDKs:

• Spin Python SDK:add maintainers file spin-python-sdk#112
• Spin JavaScript SDK: https://github.com/fermyon/spin-js-sdk/pull/256
• Spin Rust SDK: add MAINTAINERs.md file spin-rust-sdk#32
• Spin .NET SDK: add MAINTAINERS.md spin-dotnet-sdk#59
• Spin Nim SDK: add MAINTAINERS.md fermyon/spin-nim-sdk#4

Plugins:

• Spin Plugins Index Repository: add MAINTAINERS.md spin-plugins#64
• Spin Test Plugin: add MAINTAINERS.md spin-test#112
• Spin Cloud GPU Plugin: add maintainers file fermyon/spin-cloud-gpu#22

Triggers:

• Spin Command Trigger: add maintainers file spin-trigger-command#40
• Spin SQS Trigger: add maintainers file spin-trigger-sqs#42
• Spin Cron Trigger: add maintainers file spin-trigger-cron#20