Implement hex web auth flow

[starbelly]

Background

Per #18 we have agreed to move forward with implementing a web auth token flow implemented in hex.pm and utilized by mix hex and rebar3_hex.

Tasks

The following tasks must be completed to resolve this issue.

• Create an issue to get this added to the gsoc 2021 roster (https://github.com/erlef/gsoc)
• Complete action item Unify registration user experience across mix and rebar3_hex #11
• Create an issue on hexpm to align on flow and low level specifications
• Create an issue on hex_core to support the new api that will be described and detailed in the above task
• Create an issue on mix hex for working the new hex_core api offerings into to mix hex
• Create an issue on rebar3_hex for working the new hex_core api offerings into rebar3_hex
• Work with gsoc mentees and mentors to resolve all issues above (sans item 1)

Related agenda items, issues, and pull requests.

[starbelly]

Just to note, I've neglected this, but purposefully. There should be a new mentors group and our plans for GSOC for these things might do well in there. That said, I wouldn't want to wait a long time before doing this.

[wojtekmach]

I have added an issue to Hex.pm: hexpm/hexpm#1024 that hopefully gives some more context. It doesn't have a detailed specification, such would hopefully be created as part of working on a proof-of-concept.