[Arista] Add ability to set smbus bus_speed through platform_manager

[huyc-arista]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Add ability to optionally specify i2c bus speed in platform_manager config.

Changes:

• Add optional busFreqHz to I2cAdapterConfig and I2cAdapterBlockConfig thrift structs
• Thread busFreqHz through PciExplorer and Utils to ioctl bus_freq_hz
• Missing piece: scd-smbus driver needs to be implemented to handle bus_freq_hz and
  create buses at the specified speed

Test Plan

At Arista we have tested setting icecube800bawn platform_manager config to set OSFP buses to 100khz/400khz
and with scd-smbus driver changes buses are created successfully at the correct speed