Add composer `provide` attributes

[xepozz]

https://getcomposer.org/doc/04-schema.md#provide

Packages to provide:

• https://packagist.org/providers/psr/event-dispatcher-implementation
• https://packagist.org/providers/psr/container-implementation
• https://packagist.org/providers/psr/log-implementation
• https://packagist.org/providers/psr/simple-cache-implementation

[vjik]

It's not need, because package must use only for development. Development package must not solve dependecy of applications. Also test-support will be in lists of Packagist as implementation, but it's cannot use for production.

PS If we resolve don't add provide, then need add explination about it to readme.

[samdark]

We currently have these for solving "installability" issue:

• https://github.com/yiisoft/psr-dummy-provider
• https://github.com/yiisoft/dummy-provider

If we'll add provides directly in this package, in the following typical situation composer won't complain that the app can't actually work without the implementation while we're developing locally but app won't be installable with --no-dev in production:

app
  require
    yiisoft/log
      require
        log-implementation
      require-dev
        yiisoft/psr-dummy-provider <-- allows installing and testing yiisoft/log separately
                                       but doesn't prevent composer to complain about missing
                                       production implementation
  require-dev
    yiisoft/test-support <-- if we add provides, it covers log-implementation dependency
                             so composer would not warn us