fix: Support reading from files that have an UTF-8 Byte Order Mark

[ldemidov]

Adds support for reading files with UTF-8 BOM. This is commonly created by Windows text editors and should be skipped because serde deserialization will not handle those bytes.

We have encountered this issue with our Windows customers who may create UTF-8 BOM files without their knowledge. Although we fixed it with a custom FileSource implementation, it would be nice to have this in the upstream to help others who may run into this issue.

This PR came from discussion in #565
Unlike that PR, this one handles only UTF-8 Boms, and not other encodings, and does not pull in any new dependencies.

• Adds a test with a UTF-8 BOM text file.
• Updates FileSourceFile to skip the 3 BOM bytes if they are detected.

[coveralls]

Pull Request Test Coverage Report for Build 16156062432

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 64.77%

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Totals
Change from base Build 16154172080:	0.1%
Covered Lines:	945
Relevant Lines:	1459

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💛 - Coveralls