Add HTTP authentication support

[guillermodotn]

Closes #237

Add HTTP authentication to the localize command so it can download files from servers that require credentials (e.g. Pulp).

Authentication methods

Method	CLI flags	Env vars	Precedence
Bearer token	--http-token	PRODUCTMD_HTTP_TOKEN	Highest
HTTP Basic	--http-username + --http-password	PRODUCTMD_HTTP_PASSWORD	Middle
netrc	--netrc-file (optional)	PRODUCTMD_NETRC_FILE	Lowest (implicit)

Implementation decisions

• stdlib only — uses netrc, base64, urllib.parse, and urllib.request from the standard library. A library like requests would arguably be more robust, but keeping it stdlib-only avoids adding a runtime dependency for what is ultimately straightforward HTTP header handling.
• Cross-origin redirect safety — Authorization header is stripped on cross-origin redirects (matches curl behavior). Port normalization handles None vs explicit default port (443/80).
• Credential validation — rejects CR/LF in tokens, usernames, and passwords to prevent header injection.
• Mutual exclusion — --http-token and --http-username/--http-password cannot be combined; username and password must be provided together. Enforced at both CLI and library level.

[lubomir]

Should the retries be updated not not retry on auth errors? It's not a big deal, but if the credentials are wrong, retry won't help and the user just has to wait longer.