feat: Pruning - expose cli command

[Jonathansumner]

Includes a new additional command prune within the fetch app

• Allows users to specify which blocks are stored locally on disk
• E.g. fetchd prune --home './' --app-db-backend 'goleveldb' --pruning 'custom' --pruning-keep-recent 100 -- pruning-keep-every 10, --pruning-interval 10

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 8a329a1):

https://fetch-docs-preview--pr327-fix-pruning-g4a38nzh.web.app

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