Release v2.0.0

API spec with changes required for Altair upgrade. Please note that some endpoints are deprecated and might not be supported by client teams anymore.

Auto-generated changelog:
Merged pull requests:

• Update description of fork schedule. #159 (mcdee)
• Contribution_and_proof event #158 (mcdee)
• Move description of dependent_root into new paragraph #157 (rkapka)
• Mark old endpoints as deprecated #156 (mpetrunic)
• Move description of dependent_root into new paragraph #155 (rkapka)
• Clarify spec endpoint values. #154 (mcdee)
• remove route on server url #152 (rolfyone)
• add syncnets to identity metadata for altair #150 (rolfyone)
• allow v2 block api to accept content type header #149 (rolfyone)
• Remove extra array wrapping the sync committees response #146 (rolfyone)
• add a description for the until_epoch field #143 (rolfyone)
• POST /eth/v1/beacon/blocks for altair #142 (rolfyone)
• add GET /eth/v2/validator/blocks/{slot} #140 (rolfyone)
• GET /eth/v1/beacon/states/{state_id}/sync_committees #139 (rolfyone)
• GET /eth/v1/validator/sync_committee_contribution #138 (rolfyone)
• POST /eth/v1/validator/contribution_and_proofs #137 (rolfyone)
• add POST /eth/v1/validator/sync_committee_subscriptions #136 (rolfyone)
• add POST /eth/v1/beacon/pool/sync_committees #135 (rolfyone)
• add /eth/v1/validator/duties/sync/{epoch} #134 (rolfyone)
• Create v2 of getState, which supports an Altair state object being returned #130 (rolfyone)
• proof of concept altair blocks #129 (rolfyone)