Add support to CORE API registered API KEY

References:
https://api.core.ac.uk/docs/v3#section/Rate-limits

* Registered Personal users: 10,000 tokens per day, maximum 10 per minute
* Registered Academic (Supporting / Sustaining members) and Non-academic users: 200k tokens per day
* Monitor your current api limit by looking for our customised HTTP headers: `X-RateLimitRemaining`, `X-RateLimit-Retry-After`, `X-RateLimit-Limit`

What needs to be done here:

- [ ] Read the API Key from the env var
- [ ] If there is a key, increase the rate limits to a maximum of 10 per minute
- [ ] If there is a key, check the headers, calculate if it is close to the limit and show warning messages