bios: Implement BIOSStringAttribute

Implement BIOSStringAttribute, most of the code is copied from
bios/bios_parser.cpp.

Implement SetAttrValueOnDbus and constructEntry for string attribute

Signed-off-by: John Wang <wangzqbj@inspur.com>
Change-Id: Ic7c6b35d32738d698d7649f97cb7843606b8a2ba
diff --git a/test/meson.build b/test/meson.build
index a7bd88f..827f9fc 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -20,6 +20,7 @@
   'libpldmresponder_base_test',
   'libpldmresponder_bios_test',
   'libpldmresponder_bios_attribute_test',
+  'libpldmresponder_bios_string_attribute_test',
   'libpldmresponder_pdr_state_effecter_test',
   'libpldmresponder_bios_table_test',
   'libpldmresponder_platform_test',