Add optional custom format specifiers to DDLogMessageFormat

[696GrocuttT]

It would be helpful to be able to specify custom format strings when using string interpolation to generate messages. This is especially true when handling floating point numbers. At first sight it looks like this could be done by changing the methods in
DDLogMessageFormat.StringInterpolation from this style:

    @inlinable
    public mutating func appendInterpolation(_ flt: Float) {
        storage.addValue(flt, withSpecifier: "%f")
    }

to this style:

    @inlinable
    public mutating func appendInterpolation(_ flt: Float, specifier: String = "%f") {
        storage.addValue(flt, withSpecifier: specifier)
    }

This would enable formatting control when using string interpolation like this:

    DDLogDebug("my float value is \(myFloat, specifier: "%.2f")")

[ffried]

Unfortunately it's not that easy. With your proposed solution, the following code would be accepted by the compiler:

DDLogDebug("Some float value: \(someFloat, specifier: "whatever")")

Which would then lead to a runtime crash.

The proper way would be to add fully typed format specifiers, similar to what OSLog does.

In the meantime, I'd recommend using FormatStyle if possible. This will lead to the parameter being recorded as string, though. However, since we're not (yet) doing much with the concrete values (except for recording the format), that's fine for now.