MINOR: ech: introduce an ech_store type and helpers in load_echkeys()

MINOR: ech: introduce an ech_store type and helpers in load_echkeys()

Alias OSSL_ECHSTORE as ech_store, and wrap OSSL_ECHSTORE_new(),
OSSL_ECHSTORE_free(), SSL_CTX_set1_echstore() and the per-file PEM
loading loop behind ech_store_new(), ech_store_free(),
ech_store_set_ctx() and ech_store_load_file(). Use them in
load_echkeys() instead of the OpenSSL calls directly.

ech_store_load_file() no longer reads strerror(errno) to explain a
BIO_new_file() failure: that function records the fopen() failure
reason in the crypto library's error queue, not in errno.
load_echkeys() already drains that queue into *err on any failure, so
just keeping the message filename-only here is enough to get the real
reason appended.

This is pure preparation with no functional change otherwise: a
following commit will give ech_store and these four helpers an
AWS-LC-specific body, so that load_echkeys() can be shared between the
two SSL libraries.