Create Fixed size BIGNUMs refactoring branch

[t8m]

name: feature/ossl_fn
design PR link: openssl/openssl#28522
Completion estimate: July 2026
Resources: @mattcaswell @levitte @t8m

[t8m]

@mattcaswell @levitte @t-j-h @arapov Please approve.

[mattcaswell]

Approved

[levitte]

Approved

[t8m]

Branch feature/ossl_fn created.