Move AdvancedHMC support to an extension

[sethaxen]

Currently AdvancedHMC support is handled with Requires instead of in an extension solely so that we can expose our own subtype of AdvancedHMC.AbstractMetric to the user easily. This PR moves the definition of that subtype to an extension and adds a new approach for constructing this metric (using AdvancedHMC.AbstractMetric(...)). While previously the metric was documented as internal, since we did guide users to using it, I've marked this change as breaking.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.38%. Comparing base (e67d60a) to head (90f8ec2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/PathfinderAdvancedHMCExt.jl	94.11%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #263       +/-   ##
===========================================
+ Coverage   81.91%   92.38%   +10.47%     
===========================================
  Files          13       13               
  Lines         586      578        -8     
===========================================
+ Hits          480      534       +54     
+ Misses        106       44       -62     

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