[Chronik] Add Tx.size to protobuf

Summary:
Adds the serialized tx size to the Tx protobuf message. We do this by using the ser_len method on BitcoinSer.

We could use some fancy data_pos delta calculation instead without having to iterate through the tx, but this is error prone and ser_len is quite cheap already.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13859