MdeModulePkg/HiiDatabaseDxe: Add string question load default support.

[poloaegis83]

Add string question load default support
load default data from PCD PcdNvStoreDefaultValueBuffer.

Description

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• Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
• Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

In bios setup page, modify a string questions
than trigger a load default process,
string question items should load default value success.

Integration Instructions

N/A