🗺 `clientLoader` and `clientAction`

[ryanflorence]

Discussed in #4950
Related proposal: #2853

@ryanflorence to write more details in here for the API

Docs need to be clear the risk of breaking your app with this API is high.

[OnurGvnc]

for reference, the relevant section in Roadmap Planning # 4; https://youtu.be/4Q1P8T27UBo?t=3230

[jacob-ebey]

Initial work on this #5543

[ryanflorence]

With RSC I think we have a different path forward here, moving back to the backlog for now.