Finishing TODO in pingora-header-serde for checking if path is a file + avoiding panics

[omavashia2005]

In

pingora/pingora-header-serde/src/dict.rs

Lines 23 to 30 in e6e677f

	pub fn train<P: AsRef<std::path::Path>>(dir_path: P) -> Vec<u8> {
	// TODO: check f is file, it can be dir
	let files = fs::read_dir(dir_path)
	.unwrap()
	.filter_map(|entry| entry.ok().map(|f| f.path()));
	dict::from_files(files, 64 * 1024 * 1024).unwrap()
	}

The TODO can be easily completed and the return type can be changed to Result<Vec<u8>, Error> to account for invalid paths (and avoid panics on unwrap).

I will be making a PR soon after this issue is published - I made the minimum required changes and all the tests in the same file pass with cargo test from the subdirectory and cargo test -p pingora-header-serde from root.

Thanks!

Om