Implement agent initiated downstream registration

[Danil-Grigorev]

Fleet provides 2 alternative approaches to cluster registration:

1. Manager initiated: Currently supported, CAPI creates cluster secret, CAPAF generates the cluster resource. Requires API server accessibility from the management cluster.
2. Agent initiated: Supported on the manual basis Add support for agent initiated downstream cluster registration #39. Does not require to have direct connection from management to the downstream (child) cluster.

Addon provider is in the position to simplify and automate the flow for ClusterClass based Clusters. Using the Topology Mutation Hook, may allow to extend post<BootstrapProvider>Commands API data field with the necessary commands to deploy the fleet agent in the newly created CAPI Cluster during bootstrap procedure.

Thanks to @richardcase for bringing this to the table.

[Danil-Grigorev]

I drafted the thoughts on how it can be implemented here: https://hackmd.io/@bldpkRSnSNqZj73Y_EV0dQ/By6M6fz7R

[Danil-Grigorev]

I’ve brainstormed the ideas some more and left a proposal for the CAAPF: https://hackmd.io/@UJaEY_gMRYSWN1nvVVscxQ/Sk5gKsSE0