Make default permissions configurable

`0755` is always used when invoking `os.MkdirAll`. Wandering if this should be configurable, at least in a global way:

E.g.
- https://github.com/mark3labs/mcp-filesystem-server/blob/main/filesystemserver/handler.go#L747
- https://github.com/mark3labs/mcp-filesystem-server/blob/main/filesystemserver/handler.go#L995
- 
```go
...
if err := os.MkdirAll(destDir, 0755); err != nil {
...
```
