Refactor message file structure

**Is your feature request related to a problem? Please describe.**
There are many (unrelated) message definitions in a single file.

**Describe the solution you'd like**
Group messages by similar purpose .

- [ ] Coin-specific messages in `messages-${COIN}.proto` file
- [ ] Bootloader messages in `message-bootloader.proto` file
- [ ] Generic reusable messages in `message-common.proto` file
- [ ] Messages for cryptographic operations in `message-crypto.proto` file
- [ ] Debug messages in `message-debug.proto` file
- [ ] Management messages in `message-management.proto` file
- [ ] Web authentication messages in `message-webauthn.proto` file
- [ ] Message integer IDs in `message.proto` file
- [ ] Script to merge them into a single file

**Describe alternatives you've considered**
Keeping a single file is unmanageable not an option

**Additional context**
Based on [Trezor definitions](https://github.com/trezor/trezor-firmware/tree/master/common/protob).

Start by locating the current single .proto message file and the build entry point that consumes it. Use the listed message-${COIN}.proto, message-bootloader.proto, message-common.proto, message-crypto.proto, message-debug.proto, message-management.proto, message-webauthn.proto, and message.proto files as the target structure. Done means the definitions are grouped and a script merges them into the required combined file.

Written by the indexing model from the issue text.