Make healthchecks for remote workloads optional

[dmjb]

Previously they were added, then disabled due to problems which were encountered in the wild. Allow them to be turned on with the TOOLHIVE_REMOTE_HEALTHCHECK env var. This approach was taken instead of adding variables to the CLI, API and run config since we want to make them mandatory in future, and I want to avoid adding options which will be redundant later.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.90%. Comparing base (e6d3bde) to head (830a7a1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/transport/http.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3462      +/-   ##
==========================================
- Coverage   64.90%   64.90%   -0.01%     
==========================================
  Files         394      394              
  Lines       38402    38408       +6     
==========================================
+ Hits        24926    24929       +3     
- Misses      11530    11533       +3     
  Partials     1946     1946              

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