MatDatePicker: Support custom styling of individual days

[ryanrborn]

Bug, feature request, or proposal:

Proposal/Feature Request

What is the expected behavior?

Custom styling of individual days (including colors and format)

What is the use-case or motivation for changing an existing behavior?

Business needs require highlighting certain days based on external factors. Easy example would be to show weekends vs weekdays differently.

Is there anything else we should know?

Could be a resolution to #13817 as well.

To fit my needs quickly I made these changes and I'm using a build of that in my application. I realize it's not ideal for the larger use case, so I'm willing to work on what would be that ideal. I'm assuming something along the lines of an ng-template or an entirely new component like how the custom header is implemented.

[ryanrborn]

This was solved with #13971

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