Fix excessive load when bootstrapping P2P peers from servers configured in a join loop

[brandond]

Proposed Changes

• Add cache to spegel p2p peerinfo handler
  libp2p may make a large number of bootstrap calls during startup; serve nodes from cache to avoid excessive CPU usage.
• Move delegating auth middleware into common package and add MaxInFlight
  Adds maximum in-flight request limits to agent join and p2p peer info request request handlers.
• Fix chainingBootstrapper to return the first successful address list
  Avoids infinite recursion when the chain includes an agentBootstrapper with a server address that points back at this node (via join address loop or external LB)

Types of Changes

bugfix

Verification

See linked issue: #12127 (comment)

1. Start server1 with cluster-init: true.
2. Join server2 against server1.
3. Join server3 against server2.
4. Remove cluster-init: true from server1 , and add server: https://server3:6443 instead.
5. Do kubectl get --raw /v1-k3s/p2p on any one of the servers.
6. Note that the servers are now stuck in a loop requesting addresses from each other. This doesn't affect availability, but it does eat up a fair bit of CPU time. If debug logging is enabled, you can see a stream of Serving p2p peer addrs message in the logs.

Testing

Linked Issues

• Excessive load when servers configured in loop with embedded-registry (spegel) enabled #12127

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 28.98551% with 49 lines in your changes missing coverage. Please review.

Project coverage is 45.20%. Comparing base (d824d5d) to head (a69a65f).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/server/auth/auth.go	19.51%	33 Missing ⚠️
pkg/spegel/bootstrap.go	0.00%	8 Missing ⚠️
pkg/cluster/https.go	78.57%	2 Missing and 1 partial ⚠️
pkg/spegel/spegel.go	0.00%	3 Missing ⚠️
pkg/agent/https/https.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12154      +/-   ##
==========================================
+ Coverage   45.13%   45.20%   +0.06%     
==========================================
  Files         189      188       -1     
  Lines       19212    19210       -2     
==========================================
+ Hits         8672     8683      +11     
+ Misses       9305     9294      -11     
+ Partials     1235     1233       -2     

Flag	Coverage Δ
e2etests	35.70% <28.98%> (+0.06%)	⬆️
inttests	34.79% <28.98%> (-0.01%)	⬇️
unittests	19.82% <13.04%> (+0.03%)	⬆️

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