Add stall detection

[jack-berg]

Resolves #1527.

Some notes:

• Adds a feature which detects slow transactions and reports as SlowTransaction analytic events
• Implementation hooks into transaction listener to maintain map of open transactions. Before harvest, it scans the map of open transactions and identifies the slowest that has been open longer than a threshold. It extracts metadata about that transaction, notably including the stacktrace of the thread that initialized it, and emits an event.
• The implementation has a mechanism preventing the same transaction to be reported multiple times.
• Disabled by default
• Threshold is configurable, and defaults to 1s (i.e. any transaction open for longer than 1s is a candidate to be reported).

[codecov-commenter]

Codecov Report

Merging #1536 (b58cfd2) into main (2c20813) will increase coverage by 0.01%.
Report is 25 commits behind head on main.
The diff coverage is 84.68%.

@@             Coverage Diff              @@
##               main    #1536      +/-   ##
============================================
+ Coverage     70.60%   70.61%   +0.01%     
- Complexity     9745     9778      +33     
============================================
  Files           813      816       +3     
  Lines         39264    39411     +147     
  Branches       5962     5975      +13     
============================================
+ Hits          27723    27832     +109     
- Misses         8856     8879      +23     
- Partials       2685     2700      +15     

Files	Coverage Δ
...ava/com/newrelic/agent/config/AgentConfigImpl.java	87.13% <100.00%> (+0.11%)	⬆️
...relic/agent/config/SlowTransactionsConfigImpl.java	100.00% <100.00%> (ø)
...com/newrelic/agent/service/ServiceManagerImpl.java	77.85% <100.00%> (-0.13%)	⬇️
...rvice/slowtransactions/SlowTransactionService.java	82.65% <82.65%> (ø)

... and 19 files with indirect coverage changes

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[jack-berg]

The build failures appear to be test flakes?

[jtduffy]

Duplicated this PR in #1542.