OvmfPkg/TlsAuthConfigLib: configure trusted CA certs for HTTPS boot

OvmfPkg/TlsAuthConfigLib: configure trusted CA certs for HTTPS boot

Introduce TlsAuthConfigLib to read the list of trusted CA certificates
from fw_cfg and to store it to EFI_TLS_CA_CERTIFICATE_VARIABLE.

The fw_cfg file is formatted by the "p11-kit" and "update-ca-trust"
utilities on the host side, so that the host settings take effect in guest
HTTPS boot as well. QEMU forwards the file intact to the firmware. The
contents are sanity-checked by NetworkPkg/HttpDxe code that was added in
commit 0fd13678a681.

Link TlsAuthConfigLib via NULL resolution into TlsAuthConfigDxe. This sets
EFI_TLS_CA_CERTIFICATE_VARIABLE in time for both
NetworkPkg/TlsAuthConfigDxe (for possible HII interaction with the user)
and for NetworkPkg/HttpDxe (for the effective TLS configuration).

The file formatted by "p11-kit" can be large. On a RHEL-7 host, the the
Mozilla CA root certificate bundle -- installed with the "ca-certificates"
package -- is processed into a 182KB file. Thus, create
EFI_TLS_CA_CERTIFICATE_VARIABLE as a volatile & boot-time only variable.
Also, in TLS_ENABLE builds, set the cumulative limit for volatile
variables (PcdVariableStoreSize) to 512KB, and the individual limit for
the same (PcdMaxVolatileVariableSize) to 256KB.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>