config: Introduce functions to write non-standard file

config: Introduce functions to write non-standard file

Introduce two new functions corresponding to "git_config_set" and
"git_config_set_multivar" to write a non-standard configuration file.
Expose these new functions in cache.h for other git programs to use.

Helped-by: Jeff King <redacted>
Helped-by: Jonathan Nieder <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>