feat: Add from_borrowed() constructor

[zharinov]

Adds from_raw_parts() for constructing models from pre-parsed components, from_pretrained() now delegates to it.

Also fixes a bug where loading would fail if the tokenizer doesn't define an unk_token (not all tokenizers have one).

[Pringled]

Thanks for making this PR @zharinov! This is a nice functionality to have I think, and good catch about the unk_token. I have two small (but nice to have) improvements; if you could implement those this is good to go. Thanks for updating the tests as well 👍

[Pringled]

@zharinov one additional comment, could you also run clippy to fix the formatting issues?

[zharinov]

Hey,

I wanted to support zero-copy initialization with include_bytes!, yet missed the array clone in my own code 🤦‍♂️

The second attempt transforms StaticModel to use Cow, allowing for both owned and static scenarios. This adds some performance penalty, but hopefully it's minimal thanks to CPU branch prediction.

Also, I applied the suggestion for unk_token.