Prepare C-language infrastructure for ISO:SQL

[mkgrgis]

No SQL behaviour changes.

Preparing to implement SQL behaviour from table discussed in https://github.com/pgspider/sqlite_fdw/issues/66#issuecomment-1504830541

This PR include:

• improve naming;
• add function for human readable error message about SQLite data value affinity;
• change return type for sqlite_convert_to_pg from Datum to NullableDatum for future proper uuid processing and other special cases such as void text affinity in data for int column;
• add TODO comments about PostgreSQL database encoding in proper places;
• reduce double calculating for two functions: sqlite3_column_bytes and sqlite3_column_bytes for the same data value;
• add draft for human readable error message about SQLite data value affinity;
• etc.

[mkgrgis]

Thanks, @nxhai98 -san! I have fixed some problems in https://github.com/mkgrgis/sqlite_fdw/commit/74bcfeaf61fd0b5db579995e4a7fcd7cb8263a08

[mkgrgis]

I add some more comments. Could you please help me check it again?

Yes. Thanks, @nxhai98 !

[t-kataym]

@mkgrgis
@nxhai98's confirmation has completed. We can accept the PR. Can I merge it?

[mkgrgis]

@t-kataym , yes, you can squash and merge my PR. Reviews by @nxhai98 was very good and helpful for me! My next little PR will be about database encoding, also without breaking of any tests.

[t-kataym]

@mkgrgis , Thank you for your reply and development. I will merge it.