Add TrustedModuleRequiredToBoot getter function

TrustedModuleRequiredToBoot is a Redfish ComputerSystem v1_14_0
property, determining if a working TPM is required in order to boot
the host. The TPM Required property is mapped to the "TPMEnable" D-Bus
property. The possible values for the Redfish property are "Required"
and "Disabled".

This commit will add the GET operations to the
TrustedModuleRequiredToBoot Redfish property.
(See https://gerrit.openbmc-project.xyz/c/openbmc/bmcweb/+/44044


for the PATCH operations for this property).

Testing:

1) Tested on HW: I manually tested retrieving the property:
     $ curl -k https://${bmc}/redfish/v1/Systems/system
{
  "@odata.id": "/redfish/v1/Systems/system",
  "@odata.type": "#ComputerSystem.v1_14_0.ComputerSystem",
...
 "Boot": {
    "AutomaticRetryAttempts": 3,
    "AutomaticRetryConfig": "RetryAttempts",
    "AutomaticRetryConfig@Redfish.AllowableValues": [
      "Disabled",
      "RetryAttempts"
    ],
    "BootSourceOverrideEnabled": "Disabled",
    "BootSourceOverrideMode": "Legacy",
    "BootSourceOverrideTarget": "None",
    "BootSourceOverrideTarget@Redfish.AllowableValues": [
      "None",
      "Pxe",
      "Hdd",
      "Cd",
      "Diags",
      "BiosSetup",
      "Usb"
    ],
    "RemainingAutomaticRetryAttempts": 3,
    "TrustedModuleRequiredToBoot": "Required"
  },
...

TrustedModuleRequiredToBoot switches between 'Required' and 'Disabled'
values when the relevant  dbus property is changed.
(D-Bus Interface: xyz.openbmc_project.Control.TPM.Policy)
(D-Bus Property: TPMEnable)

2) Redfish Validator Testing: Tested on ComputerSystem v1_14_0 schema
    Validator Test everything passed and 0 failures.

Signed-off-by: Ali Ahmed <ama213000@gmail.com>
Change-Id: I7d0b9430e592d6d8ec95cd9090551fab802d8f54