[FEATURE] Add plugin manager to tiflash

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Feature Description
Add a plugin manager/framework to allow developers to create plugins for tiflash. Plugins would be created as separate pip installable python packages. Users could then install whichever plugins they want and tiflash would automatically pick up the installed plugins and provide access to their functionality.

Example: a BLE device plugin (tiflash-ble) that adds BLE specific functions such as:

• read_primary_ble_addr()
• read_secondary_ble_addr()
• write_secondary_ble_addr()

After installing the plugin (pip install tiflash-ble) the user would have access to those functions with their tiflash installation.
tiflash.read_primary_ble_addr(serno="L11000CE")

Why feature is requested
Having a plugin framework would allow users to extend tiflash to meet their specific needs without adding device specific code into tiflash.

Describe the solution you'd like
I think using pluggy would be best. Pluggy is used by many popular open source tools such as: pytest and tox

Each plugin would be named with the format "tiflash-<plugin name>". This would allow tiflash to pick up any plugin that was installed via pip automatically.