feat: implement "create admin session" capability

[travis]

Implement plan/create-admin-session - see storacha/console#98 for an example of this in action.

Originally #358, moving here to unstick the integration tests.

[seed-deploy]

View stack outputs

• pr374-w3infra-BillingDbStack

  Name	Value
  customerTableName	pr374-w3infra-customer
  spaceDiffTableName	pr374-w3infra-space-diff
  spaceSnapshotTableName	pr374-w3infra-space-snapshot
  usageTable	pr374-w3infra-usage

• pr374-w3infra-BillingStack

  Name	Value
  ApiEndpoint	https://g8sqe0ayve.execute-api.us-east-2.amazonaws.com
  billingCronHandlerURL	https://iovdtfijmsa5564szlxa65nk640ovtjp.lambda-url.us-east-2.on.aws/
  CustomDomain	https://pr374.billing.web3.storage

• pr374-w3infra-CarparkStack

  Name	Value
  BucketName	carpark-pr374-0
  Region	us-east-2

• pr374-w3infra-RoundaboutStack

  Name	Value
  ApiEndpoint	https://amgvbc77kd.execute-api.us-east-2.amazonaws.com
  CustomDomain	https://pr374.roundabout.web3.storage

• pr374-w3infra-SatnavStack

  Name	Value
  BucketName	satnav-pr374-0
  Region	us-east-2

• pr374-w3infra-UploadApiStack

  Name	Value
  ApiEndpoint	https://9juwmnapx4.execute-api.us-east-2.amazonaws.com
  CustomDomain	https://pr374.up.web3.storage

• pr374-w3infra-BusStack

• pr374-w3infra-FilecoinStack

• pr374-w3infra-ReplicatorStack

• pr374-w3infra-UcanFirehoseStack

• pr374-w3infra-UcanInvocationStack

• pr374-w3infra-UploadDbStack

[vasco-santos]

LGTM

[vasco-santos]

can be as follow up: would love to see part of this being actually tested via the interface exported tests so that it is easier to move from this repo to other implementations https://github.com/w3s-project/w3up/blob/main/packages/upload-api/test/handlers/plan.js