Added proper extension support to AST object.

src/lib-sieve/plugins/include/cmd-include.c

src/lib-sieve/plugins/include/ext-include-common.c

src/lib-sieve/plugins/include/ext-include-common.h

src/lib-sieve/plugins/include/ext-include-variables.c

src/lib-sieve/plugins/include/ext-include-variables.h

src/lib-sieve/sieve-ast.c

src/lib-sieve/sieve-ast.h