User-facing way to modify :activation-fn or :major-modes, and initialization-options of an existing LSP client.

[KaranAhlawat]

Is your feature related or already mentioned on the wishlist?
No.

Describe your feature here.
Currently, from my understanding, the only way to modify the major-modes in which an LSP client activates is by either a.) replicating the client creation code in your own init file, or b.) modify the locally available lsp-mode source code itself and maintaining that through the various upgrades, resolving any merge conflicts that may arise.

While this is doable, I think a user-facing way to modify activation-fn or major-mode (whichever is applicable, or maybe both) should be provided, where you could essentially "tack on" major-modes you want to activate a certain LSP in.

For instance, the lsp-emmet client only activates in certain modes. I would like it to activate in elixir-ts-mode as well (while writing inline HTML), and for that I'd need to modify the client source itself.

Similar story goes for initialization-options.