Add extension as alternative for ICredentialsByHost

[ArnaudB88]

Add extension with port, username and password parameters so they can be used from the configuration:

This is a fix for #58

[mishamyte]

@merbla, @MaximRouiller is this sink still being maintained?

This fix is pretty useful in the lot of cases. Would be nice it this pull request will be accepted.

[MaximRouiller]

I'll let @merbla handle those commits since he has merge rights to this project.

[donhuff]

@merbla @MaximRouiller +1 from me. This extension is useful to me as well as I am trying to keep all Serilog configuration in settings files.

[ptmahoney]

When might this appear in a new NuGet package? Not being able to configure the port in a config files is a big omission in today's world where ISP's are moving way from supporting port 25.

[jwillmer]

Any hope of getting this merged soon?

[nblumhardt]

Port configuration went in with 3.0; sorry about the lack of response on this @ArnaudB88, this project was low on maintainer time for a long stretch and things got shifted around a bit during the catch-up phase. Thanks again for the PR!