ssl: support Apple SecTrust configurations

- configure/cmake support for enabling the option
- supported in OpenSSL and GnuTLS backends
- when configured, Apple SecTrust is the default trust store
  for peer verification. When one of the CURLOPT_* for adding
  certificates is used, that default does not apply.
- add documentation of build options and SSL use

Closes #18703

lib/Makefile.inc

@@ -77,6 +77,7 @@ LIB_VAUTH_HFILES =      \
   vauth/vauth.h
 
 LIB_VTLS_CFILES =           \
+  vtls/apple.c              \
   vtls/cipher_suite.c       \
   vtls/gtls.c               \
   vtls/hostcheck.c          \
@@ -94,6 +95,7 @@ LIB_VTLS_CFILES =           \
   vtls/x509asn1.c
 
 LIB_VTLS_HFILES =           \
+  vtls/apple.h              \
   vtls/cipher_suite.h       \
   vtls/gtls.h               \
   vtls/hostcheck.h          \