Support the VFS API about go-sqlite

[zombiezen]

Wanted to give an update: I am slowly making some progress on this feature. I've made some decent progress on sketching out the API and a proof-of-concept implementation, but I don't have something that's ready for testing yet.

The challenge I've identified with this API is that unlike other SQLite extension APIs, there (understandably) seems to be a strong use case for wrapping other VFSes, including those that are implemented in (transpiled) C. This has led me to a somewhat split API where a Go implementation gets converted into a VFS object during registration.

// VFSImplementation is the set of parameters to [NewVFS]
// that enables creation of custom [VFS] objects.
type VFSImplementation struct {
	Name          string
	MaxPathLength int

	Open         func(name string, open VFSOpenFlags) (File, VFSOpenFlags, error)
	FullPathname func(name string) (string, error)
	ReadRand     func(p []byte) (n int, err error)
	Sleep        func(d time.Duration) error
	Now          func() (time.Time, error)
        // etc.
}

// File is the interface for files in [VFSImplementation].
type File interface {
	io.ReaderAt
	Size() (int64, error)
	io.Closer
}

// A VFS is a handle to an [operating system interface].
//
// [operating system interface]: https://www.sqlite.org/vfs.html
type VFS struct { /* ... */ }

// NewVFS creates and registers a [VFS] from a Go implementation.
func NewVFS(impl *VFSImplementation) (*VFS, error)

// FindVFS returns the VFS with the given name
// or the default VFS if the name is the empty string.
// If no such VFS exists, FindVFS returns nil.
func FindVFS(name string) *VFS

// Name returns the name of the VFS.
func (vfs *VFS) Name() string

// RegisterDefault marks the VFS as the one used by default in [OpenConn].
func (vfs *VFS) RegisterDefault() error

// Unregister removes the VFS from the global registry.
// Once a VFS has been unregistered, it must no longer be used.
func (vfs *VFS) Unregister() error

// Open opens the file with the given name.
// TODO: Add details about calling FullPathname etc.
func (vfs *VFS) Open(name string, open VFSOpenFlags) (*VFSFile, VFSOpenFlags, error)

func (vfs *VFS) FullPathname(name string) (string, error)

// and so on for VFS methods...

// VFSFile is a file returned by [VFS.Open].
type VFSFile struct { /* ... */ }

// VFSFile will have various io-related methods.