refactor(embeddings builder): update trait

[marieaurore123]

• Change ExtractEmbeddableTrait to:

pub trait Embed {
    fn embed(&self, embedder: &mut Embedder) -> Result<(), EmbedError>;
}

#[derive(Default)]
pub struct Embedder {
    pub texts: Vec<String>,
}

impl Embedder {
    pub fn embed(&mut self, text: String) {
        self.texts.push(text);
    }
}

• Update macro accordingly
• On build() method, catch errors about exceeding token limit
• Add rig::to_embed(item: impl Embed) -> Vec<String> function
• EmbeddingsBuilder.documents() must take an iterator

[cvauclair]

Closed by #120