Allow dcr with initial access token

[anestos]

This PR adds the possibility to send an initial access token to the registration endpoint, as described in https://tools.ietf.org/html/rfc7591#section-3

[codecov-io]

Codecov Report

Merging #429 into master will increase coverage by 1.14%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #429      +/-   ##
============================================
+ Coverage     83.71%   84.85%   +1.14%     
- Complexity      479      481       +2     
============================================
  Files            42       42              
  Lines          2345     2357      +12     
  Branches        233      234       +1     
============================================
+ Hits           1963     2000      +37     
+ Misses          299      274      -25     
  Partials         83       83

Impacted Files	Coverage Δ	Complexity Δ
.../java/net/openid/appauth/AuthorizationService.java	78.92% <100%> (+5.93%)	16 <1> (+1)	:arrow_up:
...ava/net/openid/appauth/AuthorizationException.java	84.35% <0%> (+10.2%)	20% <0%> (+1%)	:arrow_up:

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