Add UTF8NoBOM property to System.Text.Encoding class

[augustoproiete]

This PR adds a new property to the System.Text.Encoding class called UTF8NoBOM with an instance of the UTF8Encoding class configured to not include the byte order mark (BOM).

I opted to use the name UTF8NoBOM because it's the name used in the internal EncodingCache class that is referenced in several places throughout the codebase.

For consistency with other properties of the Encoding class such as UTF8 for example, The argument throwOnInvalidBytes is not set on the constructor and it will use the default (false). This means we wouldn't be able to replace references to EncodingCache.UTF8NoBOM with references to Encoding.UTF8NoBOM as they have different behavior.

Resolves #2136 if we want to stop here for now, unless we want to also add other similar properties together such as UnicodeNoBOM and UTF32NoBOM, etc. as suggested in #2136.

[jkotas]

Thank you for the contribution. The API needs to be approved first. https://github.com/dotnet/runtime/blob/master/docs/project/api-review-process.md describes the process we follow for adding new APIs.

I am going to close this PR until the API gets approved.