[NFC][HLSL][DirectX] Move parsing of root signature metadata to `RootSignatureMetdata`

[inbelic]

This issue tracks the non-functional change of moving all parsing logic of root signature metadata and infrastructure from DXILRootSignature to Frontend/HLSL/RootSignatureMetadata.

AC:

• Move the current logic of parsing root signature metadata into a new class interface in RootSignatureMetadata
• Update DXILRootSignature to depend on and use the metadata parsing interface provided there.