v0.2.0

Breaking changes:

• Per-issuer state directory is now created even if not configured.
  To change the prefix for default state paths, set NGX_ACME_STATE_PREFIX environment variable during build (e.g. to /var/lib/nginx).
  To disable the persistent state in configuration, use state_path off.

Features:

• The external_account_key directive allows configuring external account binding.
• Support for the tls-alpn-01 challenge. The challenge directive in the acme_issuer block now allows specifying a challenge to use.
• Account URL now can be read from a file under state path.

Bugfixes:

• Compatibility with Dogtag PKI, EJBCA, OpenBao and Vault.
• Improved logs and error reporting.
• Stability and memory usage improvements.
• Updated ngx-rust from git dependency to a released version.