ESPWeb Tools Install button manifest compatibility

[DanielBaulig]

Since the PR offer of implementing user providable firmware for espweb tools install button was rejected, I'm going to build out flashing via serial from scratch using esptool-js.

Especially for the self-hosted/customized variant of ESPHome Web App though having the "preselected" firmware option that espweb tools install button provides through it's manifest mechanism seems pretty nice. A Maker might want to provide a customized version of ESPHome Web Tools that only allows for installation of their specific given firmware.

For compatibility reasons and to make migrations easy I should consider reimplementing espweb tools install button manifests in ESPWA. Maybe it would even make sense to just build a "headless" API surface with all the espweb tools install button functionality (manifest et al), but none of the UI to be used in ESPWA. Might be useful for other projects, too. Heck, espweb tools could in theory rebase onto that and just be providing the actual install button UI.