Refactor evp_generic_do_all() to behave like evp_generic_fetch()

Refactor evp_generic_do_all() to behave like evp_generic_fetch()

This is refactored to use inner_evp_generic_fetch() without any given
name, which is just there to ensure all decoder implementations are
made into methods, and then use ossl_method_store_do_all() to list
them all.

This also adds the internal evp_generic_do_all_prefetched(), which
can be used if pre-fetching needs to be done separately from listing
all the decoder implementations, or if listing may happen multiple
times.

Fixes #15538
Fixes #14837

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15604)