Implement `JsonGenerator.writeFieldId(...)` for `YAMLGenerator`

[cowtowncoder]

With latest improvements, attempt is made to try to retain "String-type" of text values written; and so values like 100 are written quoted even if MINIMIZE_QUOTES is enabled.

But there is now (2.8+) a special case -- JsonGenerator.writeFieldId(long)-- where caller can indicate that the value in question should be considered a real integral number. While it is/was added originally to support (binary) formats that can haveMaps with non-String key types, it seems like it should also be supported by YAMLGenerator` since, well, why not.

The main/only difference should be that no quoting should be added.