Expose the TarEntry start of the data stream

[carlossanlop]

Fixes #101314

Implements the API below to allow users know where the entry's data first byte is located with respect to the parent archive stream.

namespace System.Formats.Tar;

public abstract partial class TarEntry
{
    public long DataOffset { get; }
}

cc @martinpf

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[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[carlossanlop]

Failures are known:

• 'System.Net.NameResolution.Functional.Tests' Failure #105092
• nativeaot\\SmokeTests\\Reflection\\Reflection\\Reflection.cmd fails with Access Violation #105136

[ericstj]

A couple small suggestions. Also - as I mentioned offline - can we add any offset validation to the real-world TAR files from System.Formats.Tar.TestData?

[carlossanlop]

/ba-g Failures unrelated

[ericstj]

LGTM