Add support to Semantic Scholar registered API KEY

Reference:
https://www.semanticscholar.org/product/api#api-key
https://api.semanticscholar.org/api-docs/
https://www.semanticscholar.org/product/api/tutorial#key

What needs to be done here:
- [ ] Read the API Key from the env var
- [ ] If there is a key, set the header `x-api-key` (case-sensitive)
- [ ] If there is a key, increase the rate limits to (received via email):
```
1 request per second for the following endpoints:

* /paper/batch
* /paper/search
* /recommendations

* 10 requests / second for all other calls
```