Create / revive JS injection spec and support for wallet-provider

[damip]

For the integration needs of in-app browsers of mobile wallets, we need a javascript injection specification that does the following:

• the upstream wallet injects JS code in the webpage that appends an element in a global variable listing available wallets. The added object identifies the wallet, exposes functions to interact etc...
• the wallet-provider in the webpage detects the list of all wallets declared in that global variable and makes them available

Ideally this would be compatible with all forms of JS injections: in-appbrowser, desktop browser plugin etc...

[Ben-Rey]

See #276

[SlnPons]

Blocked as we're waiting for a review and feedback from EllipX's team.