feat: add gemini embedding translation

[hustxiayang]

Description
Gemini embedding api in vertex ai : https://docs.cloud.google.com/vertex-ai/generative-ai/docs/model-reference/text-embeddings-api#generative-ai-get-text-embedding-drest

Somethings may need discussions:

• I extend the input so that it can set task type for each string/text.
• I also allow users to set a global task type for the embedding requests, which follows vendor-specific fields. This should be convenient for users, and similar to gemini api: https://ai.google.dev/gemini-api/docs/embeddings#task-types

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.73109% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (502d8ad) to head (a6c4a20).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/apischema/openai/union.go	68.11%	12 Missing and 10 partials ⚠️
...ternal/translator/openai_gcpvertexai_embeddings.go	86.77%	10 Missing and 6 partials ⚠️
internal/translator/openai_gcpvertexai.go	60.71%	10 Missing and 1 partial ⚠️
internal/endpointspec/endpointspec.go	0.00%	2 Missing ⚠️
...rnal/tracing/openinference/openai/request_attrs.go	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1625      +/-   ##
==========================================
- Coverage   83.41%   83.29%   -0.12%     
==========================================
  Files         138      139       +1     
  Lines       12447    12664     +217     
==========================================
+ Hits        10383    10549     +166     
- Misses       1431     1465      +34     
- Partials      633      650      +17     

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[hustxiayang]

/retest

[hustxiayang]

/retest

[hustxiayang]

/retest