add giga-embedding-instruct model

[ekolodin]

Checklist

• Run tests locally to make sure nothing is broken using make test.
• Run the results files checker make pre-push.

Adding a model checklist

Ref #

• I have added model implementation to mteb/models/ directory. Instruction to add a model can be found here

[Samoed]

Could you run MiraclReranking with the current MTEB version since its score format has changed? Also, could you add the implementation to the MTEB repository?