CLI Quiet Mode and Custom Progress Writer

[Gussy]

This PR adds support for custom progress writers, allowing consumers of the Go package to implement their own progress reporting while maintaining full backward compatibility.

A new (global) --quiet flag that will silence verbose output as discussed in #117

One of my use-cases is to use the go-pmtiles package in an automated extract generating service. Right now I have some hacky code to parse the ASCII progressbar and convert that into a data structure that I can use to monitor and track extract progress.

API Changes

New Public Functions

// Set custom progress writer (nil disables progress)
pmtiles.SetProgressWriter(writer ProgressWriter)

// Interfaces for custom implementations
type ProgressWriter interface {
    NewCountProgress(total int64, description string) Progress
    NewBytesProgress(total int64, description string) Progress
}

type Progress interface {
    io.Writer
    Add(num int)
    Close() error
}

// Set quiet mode to reduce verbose output
pmtiles.SetQuietMode(true)

Breaking Changes

None. This is purely additive - all existing code continues to work unchanged.

Future work

#117 also mentions slog which would be a nice change, however to implement this cleanly the public APIs would need to change from accepting an log.Logger to an slog.Logger. I chose not to include that in this PR to avoid making any breaking public API changes. There are ways to wrap and override log.Logger to not break the API, but I didn't want to go down that (messy) path without maintainer encouragement.

Overall there's a lot of CLI-like logic that could be pulled out of pmtiles/ into the main.go CLI that would improve the go-pmtiles package ergonomics. The progress writer felt like a good place to start. The next one that would be useful to me is the Show() API, to programmatically get a struct of metadata.