Implement `ext_offchain_http_request_write_body` host function

[EclesioMeloJunior]

Issue summary

• Implement ext_offchain_http_request_write_body the defined here

Other information and links

• Related to #1802
• Branch offchain-req-write-body has the partial implementation, missing the unit tests

[danforbes]

Defined in the updated spec here https://w3f.github.io/polkadot-spec/develop/sect-offchain-api.html#_ext_offchain_http_request_write_body