Add gpu-search command to search and filter GPU instance types

[theFong]

Introduces a new brev gpu-search command (also aliased as brev gpu,
brev gpus, and brev gpu-list) that allows users to search and filter
GPU instance types from the Brev API. Features include:

• Filter by GPU name (case-insensitive partial match)
• Filter by minimum VRAM per GPU (in GB)
• Filter by minimum total VRAM (GPU count * VRAM)
• Filter by minimum GPU compute capability (e.g., 8.0 for Ampere)
• Sort by price, gpu-count, vram, total-vram, vcpu, type, or capability
• Support for ascending/descending sort order

The command displays results in a formatted table showing instance type,
GPU name, count, VRAM per GPU, total VRAM, compute capability, vCPUs,
and hourly price.

Includes comprehensive unit tests for filtering, sorting, and data
processing functionality.

[theFong]

$ brev gpu-search

✅ Successfully fetched 536 GPU instance types from the API

✅ Results displayed in formatted table sorted by price (default)

$ brev gpu --gpu-name A100

✅ Returned only A100 instances (partial match, case-insensitive)

$ brev gpu -g H100

✅ Returned only H100 instances

$ brev gpu --gpu-name L4

✅ Returned L4 instances (correctly not matching L40/L40S)

$ brev gpu --sort price

✅ Sorted by price ascending (cheapest first)

$ brev gpu --sort total-vram --desc

✅ Sorted by total VRAM descending (B200x8 1536GB first)

$ brev gpu --sort gpu-count -d

✅ Sorted by GPU count descending (8-GPU configs first)

$ brev gpu --sort capability

✅ Sorted by compute capability ascending