Add builder pattern to OpenSearchVectorStore
Introduces a builder pattern for OpenSearchVectorStore configuration and
refactors the package structure to org.springframework.ai.vectorstore.opensearch
for better organization and consistency with other vector stores.
The builder pattern improves usability by:
* Providing a fluent API for configuring store instances
* Making configuration options more discoverable through method names
* Enabling better validation of configuration parameters
* Supporting optional parameters with sensible defaults
* The package refactoring aligns with the project's standard package naming
conventions and improves code organization. All constructors are deprecated
in favor of the new builder pattern to guide users toward the preferred
configuration approach.
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