Add podcast api tool to use NLP to search all podcasts or episodes. (#1833)

Use the following code to test:

```python
import os
from langchain.llms import OpenAI
from langchain.chains.api import podcast_docs
from langchain.chains import APIChain

# Get api key here: https://openai.com/pricing
os.environ["OPENAI_API_KEY"] = "sk-xxxxx"

# Get api key here: https://www.listennotes.com/api/pricing/
listen_api_key = 'xxx'

llm = OpenAI(temperature=0)
headers = {"X-ListenAPI-Key": listen_api_key}
chain = APIChain.from_llm_and_api_docs(llm, podcast_docs.PODCAST_DOCS, headers=headers, verbose=True)
chain.run("Search for 'silicon valley bank' podcast episodes, audio length is more than 30 minutes, return only 1 results")
```

Known issues: the api response data might be too big, and we'll get such
error:
`openai.error.InvalidRequestError: This model's maximum context length
is 4097 tokens, however you requested 6733 tokens (6477 in your prompt;
256 for the completion). Please reduce your prompt; or completion
length.`

docs/modules/agents/tools.md

@@ -145,3 +145,10 @@ Below is a list of all supported tools and relevant information:
 - Requires LLM: No
 - Extra Parameters: `top_k_results`
 
+**podcast-api**
+
+- Tool Name: Podcast API
+- Tool Description: Use the Listen Notes Podcast API to search all podcasts or episodes. The input should be a question in natural language that this API can answer.
+- Notes: A natural language connection to the Listen Notes Podcast API (`https://www.PodcastAPI.com`), specifically the `/search/` endpoint.
+- Requires LLM: Yes
+- Extra Parameters: `listen_api_key` (your api key to access this endpoint)