[pydantic > 2] [API] Fix `IRI` typing of `verb` and `activity` queries in `get`

[Leobouloc]

Feature Request

After Pydantic 2.0 migration, the get function in src/ralph/api/routers/statements.py typing has different values for verb and activity. In addition to that, a block was added for later coercion of these fields, although it should not be necessary. It would be nice to tidy this up.

TODO:

• Fix input type hints for verb and activities
• Remove type coercion for verb and activities within get
• Remove noqa: PLR0912 (too many branches)
• Add test for IRI as input to backend query ?