add methods to FieldLogger

[tmshn]

LogrusLogger (later renamed as FieldLogger) was introduced in #320 three years ago.

However, this interface is not updated since then; missing some methods added later.

This pr added those methods:

• WithContext introduced in Add WithContext #919
• WithTime introduced in Override time in log entries #781
• Trace, Tracef, Traceln introduced in Added TRACE level logging. #844
• Log, Logf, Logln introduced in Add Generic Log functions with level via argument #863

[tmshn]

Sorry, I saw b54cafe and now understand that this is intended...the duck typing is hard... 😢

[tmshn]

Mistakenly reopened.

#872 and #987 addresses my concern, hopes v2 makes things clear