Upstream avail adapter

[preston-evans98]

Description

This PR merges the avail adapter as implemented by @vibhurajeev. Demos and end-to-end tests using the adapter will be PR'd separately.

[codecov]

Codecov Report

Merging #905 (035035c) into nightly (7a05e1f) will decrease coverage by 1.5%.
The diff coverage is 0.0%.

Files	Coverage Δ
adapters/avail/src/spec/mod.rs	0.0% <0.0%> (ø)
adapters/avail/src/avail.rs	0.0% <0.0%> (ø)
adapters/avail/src/spec/hash.rs	0.0% <0.0%> (ø)
adapters/avail/src/spec/address.rs	0.0% <0.0%> (ø)
adapters/avail/src/spec/block.rs	0.0% <0.0%> (ø)
adapters/avail/src/spec/header.rs	0.0% <0.0%> (ø)
adapters/avail/src/verifier.rs	0.0% <0.0%> (ø)
adapters/avail/src/spec/transaction.rs	0.0% <0.0%> (ø)
adapters/avail/src/service.rs	0.0% <0.0%> (ø)

... and 2 files with indirect coverage changes

[citizen-stig]

@preston-evans98 is it possible to add at least some tests? For example for types conversion?

Ideally we would like to have tests with wiremock, including error cases and timeouts.