Lazy load credentials for legacy repositories

[axxelG]

Move loading credentials from LegacyRepository.init() separate function and call it from _get_page() and authenticated_url()
This avoids loading credentials for commands that don't use them (e. g. poetry env list). This helps to avoid annoying users that have to unlock their keyring and if such commands are called as background tasks (CI pipelines, VS-Code environment detection).

Questions and remarks from my side:

• Because of the current implementation I assumed that legacy repositories always have basic auth. If authentication is optional, I would need help to figure out possible cases and to adapt (parameterize) the tests. My pytest knowledge is very limited.
• Are you appreciating the more verbose docstring/comment on authenticated_url()? I'm fine with removing it.
• I was not sure if _set_authentication_creds() should be a private or public function. Tell me if you prefer it to become set_authentication_creds()

Resolves: #4874

• [ x ] Added changed tests for changed code.
• Updated documentation for changed code.

[axxelG]

pre-commit.ci autofix

[abn]

This code path is only used for uploads not for requests etc. see Authenticator for details.

[abn]

I suspect the spirit of this change has been implemented as a side-effect of #5518. Please comment if I am missing something.

[github-actions]

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