Tidy: Complete TrustedProxies feature

[Bisstocuz]

Background

Related issues: #2814 #2697 #2819
Related pull requests: #2832 #2692

About #2692 :

This fix has 2 problems:

1. it's not backward-compatible
2. is doesn't update related documents

#2692 (comment) by @leafduo

What I do

So I did these things below in this PR:

1. Update related documents.
2. Make Engine.TrustedProxies private, force people to use Engine.SetTrustedProxies() to set proxies.
3. Added proxies safety check and notices in Run, RunTLS, etc.
4. Completed related unit tests.

This PR should be published with #2692 .

[Bisstocuz]

Also hope you guys to help #2844 with code review.

[codecov]

Codecov Report

Merging #2887 (5b8833e) into master (1a2bc0e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2887   +/-   ##
=======================================
  Coverage   98.75%   98.76%           
=======================================
  Files          41       41           
  Lines        3063     3065    +2     
=======================================
+ Hits         3025     3027    +2     
  Misses         26       26           
  Partials       12       12           

Flag	Coverage Δ
go-1.13	98.76% <100.00%> (+<0.01%)	⬆️
go-1.14	98.59% <100.00%> (+<0.01%)	⬆️
go-1.15	98.59% <100.00%> (+<0.01%)	⬆️
go-1.16	98.59% <100.00%> (+<0.01%)	⬆️
go-1.17	98.49% <100.00%> (+<0.01%)	⬆️
macos-latest	98.76% <100.00%> (+<0.01%)	⬆️
nomsgpack	98.74% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	98.76% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
context.go	97.98% <ø> (ø)
gin.go	99.10% <100.00%> (+<0.01%)	⬆️

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[Bisstocuz]

Hello, help with code review please.
@zihengCat @leafduo @AlbinoGeek @menduo

[appleboy]

This PR should be merge to v1.7.5

cc @thinkerou

[Bisstocuz]

@appleboy
Excuse me, I fixed some typo so you need to approve again plz.

[thinkerou]

lgtm

[Bisstocuz]

@appleboy Hi, this PR can be merged.

[ItalyPaleAle]

@thinkerou @appleboy
Please note that this:

Make Engine.TrustedProxies private, force people to use Engine.SetTrustedProxies() to set proxies.

Was a breaking change, and should not have been merged in a patch version. It did break people's code.

See https://go.dev/blog/v2-go-modules :

For projects which are declared stable — at major version v1 or higher — breaking changes must be done in a new major version.