Username and password can be set using cli flags, env variables and stdin.

What type of PR is this?

Uncomment one (or more) /kind <> lines:

/kind bug

/kind cleanup

/kind design

/kind documentation

/kind failing-test

/kind feature

/kind flaky-test

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area library

/area cli

/area tests

/area examples

What this PR does / why we need it:

This PR extends the falcoctl registry auth basic command by allowing username and password to be set using the cli flags, env variables and stdin other than the interactive prompt.

See examples:

Example - Log in with username and password from command line flags:
	falcoctl registry auth basic -u username -p password localhost:5000
Example - Login with username and password from env variables:
	FALCOCTL_REGISTRY_AUTH_BASIC_USERNAME=username FALCOCTL_REGISTRY_AUTH_BASIC_PASSWORD=password falcoctl registry auth basic localhost:5000
Example - Login with username and password from stdin:
	falcoctl registry auth basic -u username --password-stdin localhost:5000
Example - Login with username and password in an interactive prompt:
	falcoctl registry auth basic localhost:5000

Which issue(s) this PR fixes:

Fixes #
#134

Special notes for your reviewer: