NetworkPkg: Define one private variable for HTTPS to set Tls CipherList.

NetworkPkg: Define one private variable for HTTPS to set Tls CipherList.

v2:
* Rename the file/variable name.

This variable (HttpTlsCipherList) can be set by any platform that want to
control its own preferred Tls CipherList for the later HTTPS session.

The valid contents of variable must follow the TLS CipherList format defined
in RFC 5246. The valid length of variable must be an integral multiple of 2.
For example, if below cipher suites are preferred:
    CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA256 = {0x00,0x3C}
    CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA256 = {0x00,0x3D}
Then, the contents of variable should be:
    {0x00,0x3C,0x00,0x3D}

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Cc: Zimmer Vincent <vincent.zimmer@intel.com>
Cc: Yao Jiewen <jiewen.yao@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>