Allow more flexible initial_params

[penelopeysm]

Currently on breaking, initial_params has to be specified as an AbstractInitStrategy. This PR makes it such that:

• NamedTuple and AbstractDict{<:VarName} are also allowed, they get wrapped in InitFromParams with an info message encouraging people to upgrade instead;
• Vector errors with a link to the docs telling people how to upgrade.

[github-actions]

DynamicPPL.jl documentation for PR #1064 is available at:
https://TuringLang.github.io/DynamicPPL.jl/previews/PR1064/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.46%. Comparing base (908d402) to head (0795c75).

Additional details and impacted files

@@             Coverage Diff              @@
##           breaking    #1064      +/-   ##
============================================
+ Coverage     82.42%   82.46%   +0.03%     
============================================
  Files            42       42              
  Lines          3812     3820       +8     
============================================
+ Hits           3142     3150       +8     
  Misses          670      670              

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