convert: make convert_attrs() and convert structs public

convert: make convert_attrs() and convert structs public

Move convert_attrs() declaration from convert.c to convert.h, together
with the conv_attrs struct and the crlf_action enum. This function and
the data structures will be used outside convert.c in the upcoming
parallel checkout implementation. Note that crlf_action is renamed to
convert_crlf_action, which is more appropriate for the global namespace.

Signed-off-by: Jeff Hostetler <redacted>
Signed-off-by: Matheus Tavares <redacted>
Signed-off-by: Junio C Hamano <redacted>