feat: add migration tool migrate keyshares from GCP to local encrypted key file

[near-bookrock]

This PR adds an option to the main CLI program to migrate key shares from GCP to local encrypted storage. The PR resolves
#245 .

[andrei-near]

I think it would make sense to avoid GCP interactions all together and have mpc cli write given keyshare to local store using provided local encryption key. Since is a time operation, users would get their current keyshare and local encryption key from GCP manually and import to local store, something like this:

$ CURRENT_KEYSHARE='{"epoch":1,"private_share":"1111111111111111111111111111111111111111111111111111111111111111","public_key":"1111111111111111111111111111111111111111111111111111111111111111"}'
$ CURRENT_LOCAL_ENCRYPTION_KEY="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
$ mpc-node import-keyshare $MY_CURRENT_KEYSHARE $CURRENT_LOCAL_ENCRYPTION_KEY

Additionally it simplifies the process as users will not have to authenticate on their non-GCP servers in order to fetch data from GCP Secret Manager