Add OSSL_FN_CTX size helpers

[levitte]

Ref: openssl/openssl#31479 (comment)

Goal

Introduce a byte-oriented sizing model for OSSL_FN_CTX and add operation-specific OSSL_FN_{operation}_ctx_size() helpers that derive the required context allocation from the actual OSSL_FN operands, then use those helpers in the existing BN_ wrappers and tests.

Scope

• Add general OSSL_FN_CTX byte-size calculation API.
• Decide and implement the OSSL_FN_CTX_new() / OSSL_FN_CTX_new_size() migration path.
• Add ctx-size helpers next to the corresponding OSSL_FN_* declarations in include/crypto/fn.h.
• Move existing ad-hoc limb calculations from crypto/bn/*.c into crypto/fn/*.c helpers.
• Make compound helpers call each other where possible.
• Update tests to exercise the new sizing helpers and allocation API.

[levitte]

I'm planning three PRs:

• one with the base functionality, and implementations for operations the implementations of which are already merged in feature/ossl_fn
• one addressing functions added in Add the 'div' and 'mod' functions openssl#30822
• one addressing functions added in OSSL_FN: Add modular arithmetic operations [pending #30822] openssl#31479