Remove references to vendor extensions in favor of extensions or x-extensions

[earth2marsh]

During the working group 2.0 spec process, extensions were proposed to suit vendor usecases, and therefore were initially referred to as vendor extensions.

However, there are plenty of reasons to extend the spec without being a vendor, hence the name is too narrow. This proposal recommends deprecating any reference to vendor extensions in favor of extensions or x-extensions.

[IvanGoncharov]

@earth2marsh Totally agree with you.
Moreover, it motivates vendors to create their own dialects like x-ms-*(see this).
And the same true for other vendors, see #616

Please also consider adding following recommendations:

• to reuse existing extension
• create more generic extensions, and provide a public specification for them.
• add vendor namespace only if an extension is specific to this particular vendor.

[DavidBiesack]

agree dropping 'vendor' is good. (I suggest the term 'specification extensions' over x-extensions, but it's not hugely important.)

Namespaces for extension names perhaps should be addressed as a separate issue, though. Vendors or other non-vendor organizations will still likely use some x-mynamespacehere- prefix to avoid conflict. That will happen regardless of the term OAS uses (extension or vendor extension).

[fehguy]

Fixed in #657