Optimize InstanceOffer construction

[r4victor]

Part of #3479

• Replace expensive InstanceOffer.dict() and InstanceOfferWithAvailability() construction with InstanceOffer.dict and InstanceOfferWithAvailability.construct() that skips validation.
• Optimize catalog_item_to_offer by using .construct()
• Drop Resources._description deprecated root validator calculation.

Local testing:

• runpod — 0.66s => 0.03s (22x)
• amddevcloud — 2.26s => 0.85s (2.7x)
• cudo — 2.48s => 1.02s (2.4x)
• digitalocean — 1.71s => 1.31s (1.3x)
• verda — 3.27s => 1.74s (1.9x)
• lambda — 3.24s => 1.89s (1.7x)
• hotaisle — 2.50s => 1.76s (1.4x)
• vastai — 3.27s => 1.77s (1.8x)
• vultr — 3.24s => 2.30s (1.4x)
• gcp — 3.74s => 2.54s (1.5x)
• nebius — 3.27s => 2.45s (1.3x)
• crusoe — 3.98s => 2.82s (1.4x)
• azure — 5.83s => 3.11s (1.9x)
• aws — 6.58s => 3.56s (1.8x)

Notes:

• Huge improvements on backends with small/no networking component.
• On smaller cloud machines the improvements should be even more substantial.