Creating util function to validate handlers

[joaopedrocampos]

Creates a util function to validate handler and adds it on redirect since it was missing.

Also changes the hostname.startsWith('[') for hostname[0] === '[' since it's more performing.

Standardizes invalid handler error messages

Related issue: #893

[codecov-commenter]

Codecov Report

Merging #894 (e3546d8) into main (ef788ee) will decrease coverage by 0.04%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##             main     #894      +/-   ##
==========================================
- Coverage   99.62%   99.57%   -0.05%     
==========================================
  Files          26       26              
  Lines        2127     2131       +4     
==========================================
+ Hits         2119     2122       +3     
- Misses          8        9       +1     

Impacted Files	Coverage Δ
lib/client.js	99.60% <66.66%> (ø)
lib/core/util.js	99.00% <94.11%> (-1.00%)	⬇️
lib/agent.js	98.05% <100.00%> (ø)
lib/core/request.js	98.98% <100.00%> (-0.13%)	⬇️
lib/handler/redirect.js	96.92% <100.00%> (+0.04%)	⬆️
lib/pool.js	100.00% <100.00%> (ø)

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[joaopedrocampos]

@ronag I created the handler validation function with the same conditions that were on request.js, but there are other places that make a simple validation's of it, e.g:

https://github.com/nodejs/undici/blob/main/lib/client.js#L286

So I don't know if it's ok to follow this approach I did or if it's better to create many "individual validation functions" (does this makes sense?) to use them in these cases

[mcollina]

lgtm