Revive procedure for moving tenants manually between safekeepers

[arssher]

Two use cases:

1. Moving all tenants residing on one safekeeper to another.
2. Move single tenant.

"Manually" means this is safe only as long as either 1) source safekeeper is reliably offline 2) or computes using it are reliably offline forever.

Needed steps:

• Make pull_timeline production-ready #6340
• Online membership config update in walproposer #6341
• revive the script

[problame]

This week: get #6340 done by writing code & tests (tests are most of the work)

[problame]

This week:

• wrap up & merge safekeeper: remove .partial suffix on the last WAL file. #7882
• implement last remaining test in to-do list of Make pull_timeline production-ready #6340

[arssher]

Opened PR with the script:
https://github.com/neondatabase/cloud/pull/15227
will test it more, finish docs and move them to the engineering book