Also separate `StacksPayload` and `BitcoinPayload` into separate types
for easier client-side inference.
Bump version to 1.8.0

When a Chainhook is triggered and we send a payload to a user, we filter
some values based on user settings (`include_proof`, `include_inputs`,
etc). This helps to reduce the payload size that is sent over the wire.

However, when we created this payload, we were omitting these fields
completely even though the corresponding type does not have those fields
as optional. This makes using our SDK types for deserialization
impossible.

This PR adds in those fields in all cases and sets them to empty values
if filtered out.

Fixes #572

Fixes #523

chore(release): publish v1.6.0

## [1.6.0](v1.5.1...v1.6.0) (2024-05-09)

### Features

* add Bitcoin transaction index to typescript client ([#568](#568)) ([6f7eba4](6f7eba4))

### Bug Fixes

* add stacks event position to ts client ([#575](#575)) ([3c48eeb](3c48eeb))
* add starting stacks height for prometheus metrics ([#567](#567)) ([6a8c086](6a8c086))
* make bitcoin payload serialization deserializable ([#569](#569)) ([5f20a86](5f20a86))
* set `Interrupted` status for streaming predicates that fail ([#574](#574)) ([11bde53](11bde53)), closes [#523](#523)
* shut down observer on bitcoin block download failure ([#573](#573)) ([f3530b7](f3530b7)), closes [#572](#572)