Bug: Summaries of asset created and updated datetimes breaks the stac spec

[billgeo]

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Describe the bug
Currently the summaries we have specified look like this:

stac/extensions/linz/examples/collection.json

Lines 39 to 51 in 7443877

	"summaries": {
	"created": {
	"minimum": "1999-01-01T00:00:00Z",
	"maximum": "2010-01-01T00:00:00Z"
	},
	"updated": {
	"minimum": "1999-01-02T00:00:00Z",
	"maximum": "2010-01-02T00:00:00Z"
	},
	"datetime": {
	"minimum": "2015-06-23T00:00:00Z",
	"maximum": "2019-07-10T13:44:56Z"
	},

This is what the summary of the item created updated datetimes look like (when the metadata is updated, not the asset).

See discussion here. radiantearth/stac-spec#1153 and radiantearth/stac-spec#1156

Could look like this:

   "summaries": {
      "created": {
        "minimum": "1901-01-01T00:00:00Z",
        "maximum": "1920-01-01T00:00:00Z"
      },
      "updated": {
        "minimum": "1901-01-02T00:00:00Z",
        "maximum": "1920-01-02T00:00:00Z"
      }
  }

And the the summary with other properties would look something like this.

   "summaries": {
    "assets": {
      "created": {
        "minimum": "1901-01-01T00:00:00Z",
        "maximum": "1920-01-01T00:00:00Z"
      },
      "updated": {
        "minimum": "1901-01-02T00:00:00Z",
        "maximum": "1920-01-02T00:00:00Z"
      }
    },
    "platform": ["Fixed-wing Aircraft"],
    "instruments": ["EAGLE IV"],
    "created": {
      "minimum": "1999-01-01T00:00:00Z",
      "maximum": "2010-01-01T00:00:00Z"
    },
    "updated": {
      "minimum": "1999-01-02T00:00:00Z",
      "maximum": "2010-01-02T00:00:00Z"
    } 
  },

To Reproduce
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Expected behavior
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Actual behavior
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Tasks

• Check if summaries can be extended or not
• If we can, create new summary properties that relate only to assets and make mandatory
• If we can't, discuss alternatives to capturing this metadata somewhere and make mandatory
• Remove mandatory schema check for summaries having an 'updatedandcreated` datetimes [Bill confirmed this is only mandatory for published data so the mandatory schema check can be removed]

[l0b0]

Blocked by #157 (comment).

[billgeo]

This is done now. Will open a new issue if there are changes/recommendations from upstream.