feat(pallet-tfgrid): allow `hdd only` nodes to register on chain

[renauter]

Description

We want to allow registration of HDD only nodes on chain for Holochain usage.

Implemented solution:

1. Similar to what already exists for SSD, add a minimum HDD size requirement on resources.validate_hru()
2. Then, on validating, make sure node has at least 1 minimum storage capacity available

ensure!(resources.validate_hru() || ensure!(resources.validate_sru() , Error::<T>::InvalidStorageInput); 

A "reasonable" minimum HDD size requirement is set to 100 GB (same as SSD).

Related Issues:

• #967

Checklist:

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• [x] I have added tests to cover my changes.
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