Add D`CENT Hardware wallet support

**What problem are you trying to solve?**
Please support D'CENT Hardware wallet. (http://dcentwallet.com)

**Describe the solution you'd like**
D'CENT has released the [D'CENT Bridge service](https://bridge.dcentwallet.com/v2) and [dcent-web-connector](https://github.com/DcentWallet/dcent-web-connector).
These will support integration MetaMask and D'CENT hardware wallet simply.

If select 'D'CENT' and click connect button, the D'CENT Bridge Service will be open.
The D'CENT Wallet will be connected through this page.
- D'CENT Bridge Service

**Additional context**

We plan to support :
- eth-dcent-keyring : A JavaScript wrapper around [dcent-web-connector](https://github.com/DcentWallet/dcent-web-connector) libraries, to support the KeyringController used by MetaMask

Start by reviewing the KeyringController integration and the referenced eth-dcent-keyring and dcent-web-connector projects. Trace how existing hardware-wallet connections are selected and opened, then determine the integration needed so selecting D'CENT opens the D'CENT Bridge and connects the wallet. Done means D'CENT hardware-wallet support works through the bridge.

Written by the indexing model from the issue text.