tor-guardmgr: Add vanguards API skeleton.

gabi-250requested to merge
gabi-250/arti:vanguard-mgr into main

This MR

  • adds an initial API for the vanguard manager. Note this only partially addresses #1275 (closed) (the design described in the ticket includes some functions I haven't added yet). The vanguards API is behind an experimental vanguards feature because its shape is not final
  • gives CircMgr a handle to VanguardMgr (closes #1277 (closed))
  • adds a tentative VanguardConfig and VanguardParams for configuring vanguards (I'm not sure we need both VanguardConfig and VanguardParams, so we might end up unifying these later down the line)
  • adds the VanguardConfig to TorClientConfig, and sprinkles reconfigure calls throughout the code for handling VanguardConfig changes

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