Update plugin_platform_interface dependency

[collinjackson]

The dependency on plugin_platform_interface is ^1.0.1, so it is incompatible with most plugins, which depend on "^2.X.Y". Could you please do one of the following:

• update it to depend on "plugin_platform_interface: >=1.0.1 <3.0.0". This will keep it compatible with 1.X version and can be done as a non-breaking change.
• update it to depend on "plugin_platform_interface: ^2.1.2" and change the name of verifyToken to verify. This will make it more compatible with future releases, but is a breaking change. (Since this plugin is a developer preview, this may be a non-issue.)

/cc @stuartmorgan