Add support for "integer" type for meta and options

[flixos90]

Currently the REST API only supports string, number and boolean for metadata and options (in other words, the type argument you specify in register_meta() and register_setting() respectively). However, JSON schema also supports integer in addition, so I think it's essential we need to support it as well (number means float, for that matter).

To make it easier for PHP developers, we could even consider supporting float (and automatically adjust that to number), since the term number doesn't sound too specific regarding its actual scalar type.

[flixos90]

38393.diff​ adds support for the integer type to be used with the REST API.

[flixos90]

38393.2.diff​ also updates the relevant register_setting() calls to use integer instead of number (all of the three fields are integers, not floats).

[rachelbaker]

Assigning to @joehoyle to review and milestone

[joehoyle]

In 39058:

REST API: Add support for "integer" type for meta and options

Previously Settings only supported "number" which meant it was possible to push floats to things like posts_per_page. This means now developers can also specify type => ineger in meta nad settings resgration.

Props flixos90.
Fixes #38393.