LIVE-826 - Learn page URL - Add environment var to enable learn staging URL

[ofreyssinet-ledger]

Context (issues, jira)

We are developing a new "Learn" screen in LLD and LLM and it's basically an iframe loading an URL.
This URL can be a staging or a prod URL.
This PR introduces an environment variable that can be used to switch between the 2 kinds (prod or staging) of URL for the Learn page.
It will help us providing builds that the e-commerce team - the team developing that Learn website - can test with either URLs.
LIVE-826

Description / Usage

Used in LLM to add a developer setting:
LedgerHQ/ledger-live-mobile@61db1cc

Expectations

• Test coverage: The changes of this PR are covered by test. Unit test were added with mocks when depending on a backend/device.
• No impact: The changes of this PR have ZERO impact on the userland. Meaning, we can use these changes without modifying LLD/LLM at all. It will be a "noop" and the maintainers will be able to bump it without changing anything.

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[ofreyssinet-ledger]

thanks @haammar-ledger !