add placeholder methods for result types to enable ide compatibility

[ajberdy]

Issue #, if available:

Description of changes:

Previously, ResultType.StateVector() for example would not be recognized as valid by the IDE, since result type constructors as ResultType classmethods are defined dynamically. Adding placeholder classmethods for established result types so the IDE recognizes them as valid.

Testing done:

existing tox and manual verification that result types work as previously (no logic was changed)

Merge Checklist

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General

• [X] I have read the CONTRIBUTING doc
• [X] I used the commit message format described in CONTRIBUTING
• [X] I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

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[codecov[bot]]

Codecov Report

Merging #196 (e0c5e85) into main (e31511f) will decrease coverage by 0.21%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   99.51%   99.29%   -0.22%     
==========================================
  Files          35       35              
  Lines        2261     2276      +15     
  Branches      295      295              
==========================================
+ Hits         2250     2260      +10     
- Misses          6       11       +5     
  Partials        5        5              

Impacted Files	Coverage Δ
src/braket/circuits/result_type.py	93.82% <66.66%> (-6.18%)	:arrow_down:

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