Migrate from httr to httr2

[florianm]

Feature

• ruODK uses httr for web requests.
• httr is no longer developed, httr2 is the recommended replacement.
• httr2 is pipe-friendly and looks like a major usability improvement over httr (which was already a really great tool).

Examples:

• https://httr2.r-lib.org/articles/wrapping-apis.html
• https://httr2.r-lib.org/articles/httr2.html

Approach

• The user experience of ruODK should not change.
• Exception: pass on improved httr2 error handling and messaging.

• Factor out request handling into a centralised, single set of wrappers around httr.
• Migrate every function to use the centralised helpers.
• Introduce httr2 as dependency and migrate first function.
• Migrate all functions.
• Drop httr as dependency.