feat: allow custom metadata key

[ogzhanolguncu]

This PR allows:

• Users to pass new metadata keys
• sessionTTL
• When adding context allows them to pass id,vector|data and metadata. When user just passes input as plain text we also put that into metadata field, but if user explicitly passes metadata, plain text will be overwritten.

[CahidArda]

I think the first condition is not correct, metadata will be like {"text": {"genre": "comedy"}}

I think there should be 4 cases:

	isText	!isText
context.metadata	{ [key]: input, ...context.metadata }	context.metadata
!context.metadata	{ [key]: input }	{}

[ogzhanolguncu]

{ [metadataKey]: context.metadata } this is same as {value: "hello world"} but key is dynamic and default is text. And, also, if user is passing plain text instead of vector and doesn't pass metadata field we'll fallback to default context.input which is plaintext field.

[CahidArda]

I hadn't realised that metadata is a string field. I guess this allows separating synthesis and retrieval texts.

I feel like people may want to include other fields other than the text itself for filtered retrieval. But I am okay with leaving this for later since it's not a must for the first release