feat(ic-ledger-types): Add AccountIdentifier.as_bytes()

[bitdivine]

Description

It is currently not possible to obtain an AccountIdentifier as bytes. It is only possible to obtain hex and then convert the hex back to bytes. This is resolved by adding an .as_bytes() method that returns a reference to the inner bytes.

Fixes #519

How Has This Been Tested?

A unit test is included that verifies that parsing bytes and returning them as bytes yields the original data.

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.
  • Rustdoc is included on the method.