Use an argument to pass the private key in ash_cmd module

[Nuttymoon]

Currently, the private key used to sign transactions with the Ash CLI is passed through the environment keyword as AVALANCHE_PRIVATE_KEY. This is not secured because we cannot hide the content from verbose logging + see Ansible docs:

environment
[...] This is not a recommended way to pass in confidential data.

Adding an avalanche_private_key argument to the module should allow us to mask the private key everywhere.