Fix ssz publishBlock sent without milestone header

[zilm13]

PR Description

It was reported that VC Teku fails to propose in connection with BN Lighthouse.
This happens because Teku sends requests without required header (see https://github.com/ethereum/beacon-APIs/blob/master/apis/beacon/blocks/blocks.yaml#L37 "Use content type header"). According to the API spec it should be used both for blind block and non-blind. Fixed with the PR.

Tested also (successfully) in Kurtosis with following configuration:

participants:
  - el_client_type: geth
    cl_type: lighthouse
    vc_type: teku
    vc_image: consensys/teku:develop
additional_services:
  - el_forkmon
  - tx_spammer
  - dora
snooper_enabled: true

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.

[StefanBratanov]

LGTM. Can possibly add changelog for bug fixes.

[rolfyone]

LGTM

[rolfyone]

this looks like we're still using the v1 block interface? I guess at some point this starts calling v3 and it needs that header too...