Further simplify config

[p-pautov]

Feature Overview

1. Support defining acme_issuer without name. There will be only 1 issuer in most configs, so there is no need to ask user for the name. The "default" issuer can be supported with following syntax: acme_certificate on | off | name; or acme_certificate [off | name];. We can even omit named issuers support entirely for the first release to check whether someone actually needs it.
2. acme_certificate should work at http scope, just like ssl_certificate, so that ACME can be enabled for all the servers. That also implies off support, as shown above.
3. Split acme_certificate into 3 separate directives, as issuer selection, certificate identifier and key type are all orthogonal concerns. And for the first release I'd suggest to omit support for custom identifiers entirely and rely only on server_name to keep only acme_certificate on|off; and acme_key ...;.
4. Remove resolver config from acme_issuer for first release to check if that level of customization is actually needed.

Alternatives Considered

No response

Additional Context

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