Allow ".esphome/platformio" storage location to be configured

**Describe the problem you have/What new integration you would like**

PlatformIO packages, platforms and cache are currently stored in `.esphome/platformio` under the config folder. When running in a container this folder is usually mounted as a volume, meaning that volume grows considerably. On my very modest setup with about a dozen devices I have a few KB of YAML configs and C++ library files, and over 500MB of data under `.esphome/platformio`.

This is especially problematic when volumes are backed by block storage devices because backups operate at the block level, meaning we have to store over 500MB to backup a few KB of YAML.

**Please describe your use case for this integration and alternatives you've tried:**

I've already changed `esphome.build_path` to move build files to ephemeral storage which redirected a few 10s of MB out of the volume, but that's <10% of the storage usage picture.

**Additional context**

Start by locating the configuration handling for esphome.build_path and the code that uses .esphome/platformio. Trace how the config-folder storage path is selected and identify the relevant tests; done means allowing the PlatformIO storage location to be configured independently while preserving the existing default.

Written by the indexing model from the issue text.