Make an ORM adapter for Casbin C/C++

[hsluoyz]

PosgreSQL is required by: https://github.com/casbin/casbin-cpp/issues/213

But I think first we need an ORM adapter to support most of common DBs.

Some ORM libs I can find are:

• https://github.com/rbock/sqlpp11 (2k stars!)
• https://github.com/BrainlessLabs/bun (94 stars)
• https://github.com/silverqx/TinyORM (only 11 stars:( )
• https://codesynthesis.com/products/odb/
• https://litesql.org/

Our requirements are:

• A modern ORM framework, no old-school thing. Source code should be hosted at GitHub
• Support all OSs like Casbin-CPP, it should not limit our user scope
• Support most popular DBs, like MySQL, PosgreSQL, SQL Server, Oracle
• Easy to use, like the Go or Java ORM
• Performance should be OK (not too slow)

[casbin-bot]

@EmperorYP7 @sheny1xuan @cs1137195420

[hsluoyz]

@divy9881 @EmperorYP7 @sheny1xuan any suggestions?