Fixes WithProxy(#658) and HttpClient(#675) issues

[PurplestViper]

Fixes 2 issues:

1. WithProxy is ignored #658
   • To avoid unnecessary socket allocation, create HttpClient when delayed until Build().
   • The injected HttpClient cannot change the Proxy, add a hint for the ignored WithProxy(IWebProxy).
2. Inject Ioc-managed HttpClient for MinioClient via constructor #675
   • HttpClient is the basic dependency of MinioClient, MinioClient (HttpClient) is still necessary to implement dependency injection.
   • Generally, the life cycle of the injected HttpClient is managed by IoC or other implementations, and there may even be other operations externally, so add an identifier to control the Dispose() behavior.

[ebozduman]

@Ruthlessist

Thank your for your contribution effort.
Your changes look great but the bottom-line is, this is the design team's decision, we cannot revive back the obsolete methods. They are deprecated and they will go away.
You are more than welcome to contribute if you have other suggestions, provided that the proposed changes will not violate MinIO requirements.

[PurplestViper]

@ebozduman Respecting the decision of the design team, although I don't think Dependency Injection and Builder patterns are opposites, the modification regarding constructors is now removed.

[ebozduman]

@Ruthlessist

There are build failures that need to be fixed.

The Linux build failure is because of the lint check.

!!!! Process Aborted !!!!
The following files do not match .editorconfig:
 * Minio/MinioClient.cs
Error: Process completed with exit code 1.

You can either run dotnet regitlint which runs on all files or dotnet regitlint -f commits -a <your-commit-id> to run only on the committed files.

[ebozduman]

@Ruthlessist
I guess you forgot to run regitlint.

[ebozduman]

LGTM
As soon as the lint changes are in we can merge.