feat: add support for importing private key armor

[zivkovicmilos]

Description

This PR is a follow up PR to #432, and it adds support for importing private key armor.

The command that's been added is:
gnokey import

The command assumes the private key armor being imported is encrypted.
Additionally, this PR also contains test cleanups for the gnokey export unit test, and changes to the KeyBase interface method ImportPrivKey to be analog to ExportPrivKey.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist (for contributors)

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

Manual tests

In addition to a unit test covering this functionality, I've tested out the command using the following:

gnokey import --name NewKey --armor-path ~/gno-test.asc

[zivkovicmilos]

@moul ping for resolving the convo

Should we move forward with merging this?