Implement parachain registration through ondemand/bulk coretime

[PierreBesson]

Now that coretime is widely available, the testnet-manager should have automation to set up:

• ondemand coretime: ability to buy 1 block with the onDemandProvider on the relay chain
• bulk coretime: abillity to buy 1 region (28 days of coretime) with broker on the coretime chain.

A guide of what extrinsics should be called is available at: https://polkadot-public.notion.site/Implementation-reference-Coretime-from-Trappist-c930796e8c884011adb1fa24ef22f77c