feat(radio): add required attribute to radio-group

[rafaelss95]

Closes #1656

[rafaelss95]

I have added required attribute in radio-button, but I just noticed that it could cause some inconsistencies, as in this example:

<md-radio-group required="true" name="any">
  <md-radio-button value="any" required="false">any</md-radio-button>
</md-radio-group>

Note that in this case it is possible to set required="true" to md-radio-group and at the same time required="false" on one or even all the radio buttons inside it.

But in this case, the required from md-radio-group will override any required attribute in inside radio-buttons due to the condition:

get required(): boolean {
  return this._required || (this.radioGroup && this.radioGroup.required);
}

It means that if the md-radio-group is required, all md-radio-button's inside it, will be required, no matter what you've set in the required attribute in md-radio-button's.

I'm just wondering if it's the correct approach. Any thoughts @jelbourn?

[tinayuangao]

LGTM

Thanks, I think your approach is correct (same as disabled behavior)

[jelbourn]

@rafaelss95 could you rebase this PR?

[rafaelss95]

@jelbourn Done.

Note that the failed checks are unrelated to this PR. Btw, it should be fixed in #5945.

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