Embed blob sidecars directly under blob_sidecars property in BlockContents

[mehdi-aouadi]

PR Description

Remove redundant blob_sidecars property name in BlockContents container. This means removing the BlobSidecars container and its BlobSidecarsSchema (Same for the signed and the signed, blinded ones).
Blob sidecars should be represented as an array under the blob_sidecars property in BlockContents.
In the current implementation there is a redundant blob_sidecars property:
BlockContents:

{
	"beacon_block": {}
	"blob_sidecars": {
		"blob_sidecars": array of BlobSidecar
	}
}

These changes fixes it to become:

{
	"beacon_block": {}
	"blob_sidecars": array of BlobSidecar
}

These changes also fix SignedBlockContents, BlindedBlockContents and SignedBlindedBlockContents

Spec reference here

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[tbenr]

looks generally ok. just a couple of renamings

[tbenr]

LGTM