Add mass erase option when using load command

[lucashutch]

Adds support for selecting mass erase from the load command by allowing --erase mass.
Some devices expose a fast path mass erase option, which is already supported in the pyocd erase command.
This PR extends this as an optional path when using the pyocd load --erase <type> command.

This updates the loader flow to:

• perform a target-level mass erase before programming
• reinitialize the target after mass erase (some targets change debug access state during a mass erase)
• disable smart/keep-unwritten behavior for the subsequent program step
• report mass erase in the programming summary

First time contributing to the repo, so let me know anything that could or should be improved with the PR.
Thanks for your time reviewing this.