Support `Path` as a field type

[mthuurne]

Services might want to configure paths, such as logging directories or locations of certificates. By supporting pathlib.Path as a field type:

• it becomes explicit that a particular field represents a file system path
• the code using the field doesn't have to convert from/to Path
• we can generate a more specific template value

[mthuurne]

I was thinking of whether there could also be a way to have the binder perform additional checks, such as whether a referenced path exists. However, there are a lot of problems with that:

• we'd need new syntax to specify the additional checks to perform, which goes against the idea of using plain dataclasses as much as possible; while I see this as more of a guideline than a hard rule, there would have to be strong advantages to breaking it
• it doesn't save the developer much work: rejecting a non-existing or non-accessible path on startup doesn't release the code using it from the obligation to handle non-existing or non-accessible paths, as the file system could have changed between the initial check and the path actually being used
• for some situations, like logging directories, the desired behavior could be to attempt to create the directory if it doesn't exist yet, but doing that as a side effect of data binding feels like overreach

So I think it's better not to do anything beyond constructing the Path object. If the developer wants to do more, __post_init__() can be used.