feat: support --custom-header flags

[jiuker]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers
under the terms of the Apache 2 license.
By creating this pull request I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 license.

Description

support header flags
Allows user use --custom-header="HeaderA:ValueA" --custom-header="HeaderB:ValueB" to sent custom header to S3.

Motivation and Context

How to test this PR?

mc ls minio --custom-header "MyHeaderA:Value" --custom-header "MyHeaderB:Value" --debug

mc: <DEBUG> GET / HTTP/1.1
Host: 192.168.101.100:9000
User-Agent: MinIO (windows; amd64) minio-go/v7.0.84 mc/DEVELOPMENT.GOGET
Accept-Encoding: zstd,gzip
Authorization: AWS4-HMAC-SHA256 Credential=minioadmin/20250210/us-east-1/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=**REDACTED**
Headera: ValueA
Headerb: ValueB
X-Amz-Content-Sha256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
X-Amz-Date: 20250210T081507Z

mc: <DEBUG> HTTP/1.1 200 OK
Content-Length: 568
Accept-Ranges: bytes
Content-Type: application/xml
Date: Mon, 10 Feb 2025 08:15:07 GMT
Server: MinIO
Strict-Transport-Security: max-age=31536000; includeSubDomains
Vary: Origin
Vary: Accept-Encoding
X-Amz-Id-2: dd9025bab4ad464b049177c95eb6ebf374d3b3fd1af9251148b658df7ac2e3e8
X-Amz-Request-Id: 1822CABA539717AC
X-Content-Type-Options: nosniff
X-Ratelimit-Limit: 4454
X-Ratelimit-Remaining: 4454
X-Xss-Protection: 1; mode=block

mc: <DEBUG> Response Time:  3.6695ms

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[ramondeklein]

Don't use globals. Although it may be tempting for a "run-once" per process tool as mc to use globals it makes unit-testing harder (and it's considered bad practice anyway).

[klauspost]

This is worse now. @ramondeklein was this what you meant?

[ramondeklein]

@klauspost No. There should be a better way to provide CLI parameters. A 200 file change PR is not acceptable. I've not the time to work on this, but it's hard to believe we need to change 200 files to send a custom header...