Added authentication functionality for the private index.

[sh4n3e]

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I have added authentication functionality for the private index.
When I use private index on Enterprise Github environment (Internal/Private Repository), Exsisted Version's falcoctl is not supported.

> falcoctl index add private_index https://raw.enterprise-gitpro.ttaallkk.top/org_name/repo_name/main/index.yaml https {{ base64.encode("Header_name:Authentication_token") }}

[poiana]

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[alacuku]

Hey @sh4n3e, thanks for the PR.

Authentication when retrieving index files could be handy in some contexts, and I thank you for bringing this up. However, we need a simpler user experience when using authentication. Furthermore, we support different backends for index files, and the solution proposed in this PR is not flexible enough to support all the existing backends or future ones.

The command should expose three new flags:

-p, --password string   index password
      --password-stdin    read password from stdin
  -u, --username string   index username

Those values should be then used for the current backend if supported.

Here is the PR that added the same feature for the authentication command: #588