feat: add method convert str to embedding model enum

[warjiang]

Describe the change
Use go-openai as part of my application, params passed to application may be string, but there is no method to convert model name to enum for embedding, so i add this helper method for convenient.

Describe your solution
Reuse the implementation of stringToEnum. If the target mapping is found in the stringToEnum mapping, use it directly. Otherwise, return the Unknown enumeration type.

Tests
ConvertStr2EmbeddingModel("text-embedding-ada-002") should return AdaEmbeddingV2

Additional context
Nop

Issue: #503

[codecov]

Codecov Report

Merging #504 (d55b3d9) into master (0d5256f) will decrease coverage by 0.58%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   97.46%   96.88%   -0.58%     
==========================================
  Files          18       18              
  Lines         827      835       +8     
==========================================
+ Hits          806      809       +3     
- Misses         15       20       +5     
  Partials        6        6              

Files	Coverage Δ
embeddings.go	93.97% <44.44%> (-6.03%)	⬇️

[vvatanabe]

@warjiang How about the following approach?
#503 (comment)