API: Use /records instead of /record for endpoints that affect multiple records

[QuantumDancer]

After implementing #597 I noticed that right now we are a little bit inconsistent with naming the API endpoints (see the docs). The bulk insert uses /records, but querying all or a subset of records still uses /record. We should make this consistent by changing all API endpoints affecting more than one record to /records. Right now these are:

• query all records (GET /record -> GET /records)
• query subset of records with a query string (GET /record?<query_string> -> GET /records?<query_string>)

When changing this, we also need to make sure the documentation is up-to-date:

• Rust client
• Python client
• API documentation
• Migration guide