feat(aws): add dimensions parameter to BedrockEmbeddings

[johnkitaoka]

Adds a dimensions parameter to BedrockEmbeddings for controlling output embedding vector size, following the pattern established by OpenAIEmbeddings.

Closes #796

Supports provider-specific API schemas:

• Titan v2: dimensions (top-level)
• Cohere v4: output_dimension (v3 does not support dimensions)
• Nova: embeddingDimension (inside singleEmbeddingParams)

Example:

from langchain_aws import BedrockEmbeddings

embeddings = BedrockEmbeddings(
    model_id="amazon.nova-2-multimodal-embeddings-v1:0",
    dimensions=256,
)