Add the NetworkOptions.__diagnostics__() function

[DilumAluthge]

See JuliaLang/julia#43834 (comment) for context.

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Example output if no relevant environment variables are set:

julia> NetworkOptions.__diagnostics__(stdout)
Relevant environment variables: [none]

Example output if one or more relevant environment variables are set:

julia> NetworkOptions.__diagnostics__(stdout)
Relevant environment variables:
  SSH_KEY_PASS:  ***
  SSL_CERT_DIR:  ***
  SSL_CERT_FILE: ***

[codecov]

Codecov Report

Merging #21 (23c86a3) into master (4d3df64) will decrease coverage by 9.32%.
The diff coverage is 44.44%.

❗ Current head 23c86a3 differs from pull request most recent head 44259fd. Consider uploading reports for the commit 44259fd to get more accurate results

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   97.63%   88.31%   -9.33%     
==========================================
  Files           4        5       +1     
  Lines         127      154      +27     
==========================================
+ Hits          124      136      +12     
- Misses          3       18      +15     

Impacted Files	Coverage Δ
src/NetworkOptions.jl	100.00% <ø> (ø)
src/diagnostics.jl	44.44% <44.44%> (ø)

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[KristofferC]

crossposting JuliaLang/Pkg.jl#2992 (comment) here.

[StefanKarpinski]

I'd rather make this an official API and not hide it with underscores. It also seems like it might be better to return a data structure, perhaps one that looks sane when printed as a TOML data structure?