feat: copy Caller to new entry if reportCaller is not set

[shallowclouds]

Copy Caller to new entry if ReportCaller is not set when log.

There are some scenarios where users want to pass the caller by themself, for example, many libraries accept a logger interface and use it to output internal log(such as SQL exec log for Gorm), many users wrap logrus to the needed interface, but the caller got by logrus is inaccuracy in this case, and need to get the caller and pass it into Entry, just throwing caller away will break wrapped loggers for these libraries.

Passing caller in Entry works util #1229 imported Dup to fix race condition issue.

[shallowclouds]

@dgsb Could I ask you for code review? Do you think this is valuable for users?(This does value for me) Thanks!

[shallowclouds]

It seems that #1215 has done this, but it may be completed without importing a new method to Entry.