Add mempool widget

[mayankchhabra]

[github-actions]

❌ Linting failed with 3 errors ❌

Thank you for your submission! This is an automated linter that checks for common issues in pull requests to the Umbrel App Store.
Please review the linting results below and make any necessary changes to your submission.

Linting Results

Severity	File	Description
❌	mempool/docker-compose.yml	/services/widget-server/environment: must be object
❌	mempool/docker-compose.yml	/services/widget-server/environment/0: must be string
❌	mempool/docker-compose.yml	/services/widget-server/environment: must match exactly one schema in oneOf

Legend

Symbol	Description
❌	Error: This must be resolved before this PR can be merged.
⚠️	Warning: This is highly encouraged to be resolved, but is not strictly mandatory.
ℹ️	Info: This is just for your information.

[github-actions]

🎉 Linting finished with no errors or warnings 🎉

Thank you for your submission! This is an automated linter that checks for common issues in pull requests to the Umbrel App Store.

[nmfretz]

Tested on arm64 and x86! Working beautifully.