Fix remote model with embedding input issue

[b4sjoo]

Description

All remote embedding model will throw exception when use embedding input instead of remote input. However Neural Search uses a hardcoded value FunctionName.TEXT_EMBEDDING when instantiating the MLInput:
https://github.com/opensearch-project/neural-search/blob/7feacd67b3c7694ff4a1c1c2b430f2447a1ed4ab/src/main/java/org/opensearch/neuralsearch/ml/MLCommonsClientAccessor.java#L293

Therefore, we removed the required string parameter in input interface to enable the dual usage of remote model. In the future, we plan to use different interface on different usage in same connector as a long term fix.

Related Issues

Resolves #3261

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[dhrubo-os]

1. Can we have any unit test which can reflect this change?
2. Is there any documentation for model interface? Do we need to update the documentation too?

[b4sjoo]

1. Can we have any unit test which can reflect this change?
2. Is there any documentation for model interface? Do we need to update the documentation too?

UT added.
Interface doc: https://opensearch.org/docs/latest/ml-commons-plugin/api/model-apis/register-model/#the-interface-parameter
No need to added because we are trying to fix the interface back to the expected behavior.