Add _prefix to p-adics

[r-mb]

Fixes #39933.
The issue was that, starting from a finite field K without a _prefix attribute, it was impossible to create a p-adic ring whose reduction was K.
I had two reports from other users stating that it was an issue for them, on my side it was a problem in the Witt vector packages: I added the relevant doctest there.

It was necessary to do a change in the create_key_and_extra_args method of finite fields. I believe it should not be too controversial. Namely, the check_irreducible and check_prime arguments are now extra arguments, and not part of the key. This move is necessary to test that two finite fields created with and without these options are considered equal.

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[r-mb]

@roed314 I am pinging you because I remember you telling me you had a colleague affected by this bug. If by any chance she would be able to check that this patch fixes her issues, it would be super wonderful.

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