Parse cluster.txt in C extension

[djianwen-vmware]

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PXF 7 will support multi-cluster feature and thus the C extension needs the functionality to know which PXF host and port to communicate with. This PR does the following:

• Create a new folder extension-common to store C source and test files shared by fdw and external-table extensions
• Create pxfconfigparser module under extension-common folder. It includes a header file, a source file and a test file.
• Add Makefileand README.md to extension-common folder for compiling the tests.
• Add pxf_deployment, pxf_cluster, pxf_service_group and pxf_host structs to extension-common/pxfconfigparser.h header file. Also add some macros in the file.
• Add functions to extension-common/pxfconfigparser.c for parsing cluster.txt of default cluster into a pxf_deployment struct instance.
  • For now, there is only one PXF cluster: default.
  • For now, the PXF_HOME is passed in as a parameter for parsing cluster.txt. We will use custom GUC instead of parameter in function later.
• Add cleanup function deployment_cleanup() to extension-common/pxfconfigparser.c to free up the memory used by pxf_deployment.
• Add unit tests for the parser

Other changes:

• Include the extension-common/pxfconfigparser.o in the Makefile of external-table and fdw.
• Identify some places where the parser may be used and add comments there. Also include the header file there.
• Add a fixture folder extension-common/test/resources and some *.txt files for testing purpose.